Conception, and Fetal Development
LEARNING OBJECTIVES
* Explain the basic principles of genetics.
* Describe the Human Genome Project.
* Describe the nurse's role in genetics.
* Examine ethical dimensions of genetic screening.
* Summarize the process of fertilization.
* Describe the development, structure, and functions
of the placenta.
* Describe the composition and functions of the amniotic
fluid.
* Identify three organs or tissues arising from each
of the three primary germ layers.
* Summarize the significant changes in growth and
development of the embryo and fetus.
* Identify the potential effects of teratogens during vulnerable periods of embryonic and fetal
development.
KEY TERMS AND
DEFINITIONS
blastocyst Stage
in development of a mammalian embryo, occurring after the morula
stage, that consists of an outer layer, or trophoblast,
and a hollow sphere of cells enclosing a cavity
chorionic villi Tiny vascular protrusions on the chorionic surface that project into the maternal blood
sinuses of the uterus and that help form the placenta and secrete human chorionic gonadotropin
chromosomes Elements
within the cell nucleus carrying genes and composed of DNA and proteins
conception
decidua
basalis Maternal aspect of the placenta made
up of uterine blood vessels, endometrial stroma, and
glands; shed in lochial discharge after delivery
embryo Conceptus from the
second or third week of development until approximately the eighth week after
conception fertilization
fetal membranes Amnion
and chorion surrounding the fetus
fetus Child
in utero from approximately the ninth week after
conception until birth
gamete Mature
male or female germ cell; the mature sperm or ovum
genome Complete
copy of genetic material in an organism implantation Embedding of the
fertilized ovum in the uterine mucosa; nidation
karyotype Schematic
arrangements of the chromosomes within a cell to demonstrate their numbers and
morphology
meiosis Process
by which germ cells divide and decrease their chromosomal numbers by one half
mitosis Process
of somatic cell division in which a single cell divides, but both of the new
cells have the same number of chromosomes as the first
monosomy Chromosomal
aberration characterized by the absence of one chromosome from the normal
diploid complement
morula Developmental
stage of the fertilized ovum in which there is a solid mass of cells resembling
a mulberry
mosaicism Condition
in which some somatic cells are normal, whereas others show chromosomal aberrations
sex chromosomes Chromosomes associated with
determination of sex: the X (female) and Y (male) chromosomes; the normal
female has two X chromosomes, the normal male has one X and one Y chromosome
teratogens Environmental
substances or exposures that result in functional or structural disability
zygote Cell
formed by the union of two reproductive cells or gametes; the fertilized ovum
resulting from the union of a sperm and an ovum
GENETICS
Genetic
causes of disease have assumed increasing importance as the incidence of
communicable diseases has decreased. For most genetic conditions, therapeutic
or preventive measures do not exist or are very limited. Consequently, the most
useful means of reducing the incidence of these disorders is by preventing
their transmission. It is standard practice to assess all pregnant women for
heritable disorders to identify potential problems (Creasy & Resnik, 1999). The incidence of chromosomal aberrations is
estimated to be 0.5% to 0.6% in newborns. Approximately 50% of miscarriages and
5% to 7% of stillbirths and perinatal deaths are
caused by chromosomal abnormalities (Lashley, 1998).
Genetic
disease affects people of all ages, from all socioeconomic levels, and from all
racial and ethnic backgrounds. Genetic disease affects not only individuals,
but also families, communities, and society. Advances in genetic testing and
genetically based treatments have altered the care provided to affected
individuals. Improvements in diagnostic capability have resulted in earlier
diagnosis and enabled individuals who previously would have died in childhood
to survive into adulthood (Lashley, 1998). The genetic
aberrations that lead to disease are present at birth but may not manifest for
many years or not at all.
Some
disorders appear more often in ethnic groups (Creasy & Resnik,
1999). Examples include Tay-Sachs disease in
Ashkenazi Jews; beta thalassemia in Italians and Greeks;
sickle cell anemia in African-Americans; alpha thalassemia
in Southeast Asians and Northern Africans; lactase deficiency in adult Chinese
and Thailanders; cleft lip and palate and Oguchi disease in Japanese; ear anomalies in Navaho
Indians; clubfoot in Polynesians; phenylketonuria in
Irish, Scots, Scandinavians, Icelanders, and Polish; cystic fibrosis in Scots
and English; Niemann-Pick disease, type D, in Nova
Scotia Acadians; and tyrosinemia in French-Canadians from
the Lac St. Jean-Chicoutimi region of Quebec (Fanaroff & Martin, 1997; Lashley,
1998).
RELEVANCE OF
GENETICS TO NURSING
Genetic
disorders span every clinical practice specialty and site, including school,
clinic, office, hospital, mental health agency, and community health settings. Because the potential effect on families and the community is
significant (Box 1), genetics must be integrated into nursing education and
practice (Lashley, 1998) (see Research box). A
genetic paradigm must be embraced by health care providers; that is, genetic
information, technology, and testing must be incorporated into health care
services (Anderson et al., 2000). Skills needed by nurses are the ability to
interview, to take a history over three generations, to recognize risk for
genetic disorders, to refer for evaluation and counseling, and to explain and
interpret the purpose and results of genetic tests (Lashley,
1998).
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Financial cost to family Decrease in planned family size Loss of geographic mobility Decreased opportunities for siblings Loss of family integrity Loss of career opportunities and job flexibility Social isolation Lifestyle alterations Reduction in contributions to their community by families Disruption of husband-wife or partner relationship Threatened family self-concept Coping with intolerant public attitudes Psychologic effects Stresses and uncertainty of treatment Physical health problems Loss of dreams and aspirations Cost to society of institutionalization or home or
community care Cost to society because of additional problems and needs
of other family members Cost of long-term care Housing and living arrangement changes From Lashley, F. (1998).
Clinical genetics in nursing practice (2nd ed.). |
Nurses
are usually the ones who provide follow-up care and maintain contact with
patients. Community health nurses can identify groups within populations that
are at high risk for illness, as well as provide care to individuals, families,
and groups (Williams, 1998). They are a vital link in follow-up for newborns who may need newborn screening. Although diagnosis and
treatment of genetic disorders require medical skills, nurses with advanced
preparation are assuming important roles in counseling people about genetically
transmitted or genetically influenced conditions. The International Society of
Nurses in Genetics (ISONG) has developed a Statement on the Scope and Standards
of Genetics Clinical Nursing Practice (Anderson et al., 2000).
Referral
to appropriate agencies is an essential part of the follow-up management. Many
organizations and foundations, such as the Cystic Fibrosis Foundation and the Muscular
Dystrophy Association, help provide services and equipment for affected
children. There are also numerous parent groups in which the family can share
experiences and derive mutual support from other families with similar
problems.
Probably
the most important of all nursing functions is providing emotional support to
the family during all as pects of the counseling
process. Feelings that are generated under the real or imagined threat posed by
a genetic disorder are as varied as the people being counseled (McGowan, 1999).
Responses may include a variety of stress reactions, such as apathy, denial,
anger, hostility, fear, embarrassment, grief, and loss of self-esteem.
GENETICS COUNSELING
SERVICES
The
most efficient counseling services are associated with the larger universities
and major medical centers where support services are available (e.g.,
biochemistry and cytology laboratories). These services consist of a group of
specialists under the leadership of a physician trained in medical genetics.
Health professionals should become familiar with people who provide genetic
counseling and places in which counseling services are available to patients in
their area of practice.
Good
reproductive decision making should be fully informed and well reasoned, with
the goals, values, and circumstances of the patients balanced with their social
and moral implications (White, 1999). Nurses can get to know the moral
understanding and ethical reasoning of patients by listening to their stories;
they will then be better able to help patients make decisions about genetic
screening and diagnostic tests that are informed and autonomous (
ETHICAL
CONSIDERATIONS
Researchers
have proposed using fetal neurologic, liver, and
pancreatic tissues to treat adults with Parkinson disease, metabolic disorders,
or head and spinal cord injury. The use of fetal tissue in research was banned
for several years, but the ban was lifted in 1993. Research involving human
stem cells shows great promise for health care advances because stem cells can
give rise to different kinds of cells, including muscle cells, heart cells,
blood cells, and nerve cells. On August 25, 2000, the National Institutes of Health
(NIH) published guidelines for research using human stem cells (NIH News
Release, 2000). Research with stem cells is controversial. The nurse involved
in genetics must keep abreast of new developments and be prepared to discuss
ethical implications with patients and other health care providers.
Most
genetic testing is offered prenatally to identify
genetic disorders in fetuses (White, 1999). When an affected fetus is
identified, termination of the pregnancy is an option. Genetic testing may be
requested for sex selection and for late-onset disorders. An ethic of social
responsibility should guide genetic counselors in their interactions with
patients (White, 1999) while recognizing that people make their choices by
integrating personal values and beliefs with their new knowledge of genetic
risk and medical treatments (Anderson, 1998).
Other
ethical issues relate to autonomy, privacy, and confidentiality. Should genetic
testing be done when there is no treatment available for the disease? When is
it appropriate to warn family members at risk for inherited diseases? When
should presymptomatic testing be done? Some who might
benefit from genetic testing choose not to have it, fearing discrimination
based on the risk of a genetic disorder. Several states have prohibitions
against insurance discrimination; other states are expected to follow their
lead (O'Connor, 1998). Until guidelines for genetic testing are created,
caution should be exercised. The benefits of testing should be weighed carefully
against the potential for harm (Giarelli &
Jacobs, 2000; O'Connor, 1998). The National Coalition for Patient Rights
advocates new efforts to protect patient privacy (
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Medical records should be
maintained as confidential and private for the purpose of clinically caring
for the patient. Patients should have the
right to determine what information in their medical records is shared with other
parties. Research should be conducted
with the freely given informed consent of patients or with blanket consent that
delegates the consent decision to a Medical Records Review Board. Employers should generally
not have access to medical records and should be barred from using them for
employment, promotion, and other personnel decisions. Systems to link or collate
private medical information using unique patient identification or Social
Security numbers should not be implemented without the explicit and informed
consent of the patient. From Clute, K. (2000). Coalition for Patients Rights fights for
genetic privacy. Mass Nurse, 70(7), 3. |
Preimplantation genetic
diagnosis (PGD) or preimplantation genetic testing
(PGT) is available in a limited number of centers. In this procedure, embryos
are tested before implantation by in vitro fertilization (IVF) (Jones & Krysa, 1998). PGD has the potential to eliminate specific
disorders in pregnancies conceived by IVF and to prevent future termination of
pregnancy for genetic reasons; no obvious detrimental effects of the procedure
have been found (Strom et al., 2000). Couples need counseling about their
options when genetic testing is done.
HUMAN GENOME
PROJECT
The
Human Genome Project began in 1990 as an international effort to map and
sequence the genetic makeup of humans; it is funded by the NIH and the
Department of Energy. There are 22 Human Genome Project Research Centers in the
An
integral part of the Human Genome Project is the Ethical, Legal, and Social
Implications (ELSI) program. This program addresses the potential that genetic
information may be used to discriminate against individuals or for eugenic
purposes. Continued awareness of and vigilance against such misuse of
information is the collective responsibility of health care providers,
ethicists, and society.
MANAGEMENT OF
GENETIC DISORDERS
At
this time, no cures exist for genetic disorders, although remedies can be
implemented to prevent or reduce the harmful effects of a few disorders.
Structural defects can sometimes be modified to produce normal or near-normal function.
Surgical therapy is employed for congenital heart defects and cosmetic defects
such as cleft lip. Advances in fetal surgery are occurring. Other conditions
are treated with product replacement (thyroid for hereditary cretinism), diet
modification (low-phenylalanine diet for phenylketonuria),
and corrective devices for missing limbs. Research is being conducted to devise
methods to influence or change genes directly by placing substitute
deoxyribonucleic acid in the cells of those with a genetic mutation, thereby
preventing or curing the disease process or relieving symptoms.
The
possibility exists that understanding embryonic stem
cells (primitive cells that can develop into all types of body tissue,
including muscles, nerves, and bones) will lead to new medical discoveries. The
successful cloning of sheep, cattle, mice, and pigs; the production of rhesus monkeys
through nuclear transfer of embryonic cells; and the isolation of stem cells
constitute breakthroughs in technology and raise other ethical questions.
Estimation of risk
The
risks of recurrence of a genetic disorder are determined by the mode of
inheritance. The risk of recurrence for disorders caused by a factor that
segregates during cell division (genes and chromosomes) can be estimated with a
high degree of accuracy by application of mendelian
principles. In a dominant disorder the risk is 50%, or one in two, that a
subsequent offspring will be affected; an autosomal recessive
disease carries a one-in-four risk of recurrence; and an X-linked disorder is
related to the child's sex, as described in the section on X-linked
inheritance. Translocation chromosomes have a high risk of recurrence.
Disorders
in which a subsequent pregnancy would carry no more risk than there is for
pregnancy alone (estimated at 1 in 30) include those resulting from isolated
incidences not likely to be present in another pregnancy. These disorders include
maternal infections (e.g., rubella, toxoplasmosis), maternal ingestion of
drugs, most chromosomal abnormalities, and a disorder determined to be the
result of a fresh mutation.
Interpretation of risk
Counselors
explain the risk estimates to patients without making recommendations or
decisions and avoid allowing their own biases to interfere. The counselor
provides appropriate information about the nature of the disorder, the extent
of the risks in the specific case, the probable consequences, and (if
appropriate) alternative options available, but the final decision to become pregnant
or to continue a pregnancy must be left to the family. An important nursing
role is reinforcing the information the families are given and continuing to
interpret this information on their level of understanding.
The
most important concept that must be emphasized to families is that each
pregnancy is an independent event. For example, in monogenic disorders in which
the risk factor is one in four that the child will be affected, the risk
remains the same no matter how many affected children are already in the
family. Families may maintain the erroneous assumption that the presence of one
affected child ensures that the next three will be free of the disorder.
However, "chance has no memory." The risk is one in four for each pregnancy.
On the other hand, in a family with a child who has a disorder with multifactorial causes, the risk increases with each
subsequent child born with the disorder.
GENETIC
TRANSMISSION
Human
development is a complicated process that depends on the systematic unraveling
of instructions found in the genetic material of the egg and sperm. Development
from conception to birth of a normal, healthy baby occurs without incident in
most cases; occasionally, however, some anomaly in the genetic code of the
embryo creates a birth defect or disorder. The science of genetics seeks to
explain the underlying causes of congenital disorders (disorders present at
birth) and the patterns in which inherited disorders are passed from generation
to generation.
Genes and chromosomes
The
hereditary material carried in the nucleus of each somatic (body) cell
determines an individual's physical characteristics. This material, called
deoxyribonucleic acid (DNA), forms threadlike strands known as chromosomes. Each
chromosome is composed of many smaller segments of DNA referred to as genes.
Genes or combinations of genes contain coded information that determines an
individual's unique characteristics. The "code" is found in the specific
linear order of the molecules that combine to form the strands of DNA.
All
normal human somatic cells contain 46 chromosomes arranged as 23 pairs of
homologous (matched) chromosomes; one chromosome of each pair is inherited from
each parent. There are 22 pairs of autosomes, which control
most traits in the body, and one pair of sex chromosomes, which determines sex
and some other traits. The large female chromosome is the X chromosome; the tiny
male chromosome is the Y chromosome. When one X chromosome and one Y chromosome
are present, the embryo develops as a male. When two X chromosomes are present,
the embryo develops as a female.
Because
each gene occupies a specific chromosome location, and because chromosomes are
inherited as homologous pairs, each person has two genes for every trait. In other
words, if an autosome has a gene for hair color, its partner
also has a gene for hair color—in the same location on the chromosome. Although
both genes code for hair color, they may not code for the same hair color.
Different genes coding for different variations of the same trait are termed
alleles. An individual with two copies of the same allele for a given trait is
said to be homozygous for that trait; with two different alleles, the person is
heterozygous for the trait.
Some
genes are dominant, and their characteristics are expressed even if another
allele is present on the other chromosome. Other genes are recessive, and their
characteristics are expressed only if they are carried by both homologous chromosomes.
When an egg and a sperm unite, the combination of alleles becomes that
individual's entire genetic makeup, or genotype. This includes all the genes that
the person carries and that can be passed to offspring. The genotype determines
the person's physical appearance, or phenotype, but this is affected by the
dominant or recessive nature of the allele.
The
pictorial analysis of the number, form, and size of an individual's chromosomes
is known as a karyotype. A karyotype
can be obtained from a blood sample that has been treated and stained to make
the replicating chromosomes visible under a microscope. The photographed chromosomes
are cut out and arranged in a specific numeric order according to their length
and shape. Fig. 1 illustrates the chromosomes in a body cell and a karyotype. Karyotypes can be used
to determine the sex of a child and the presence of any gross chromosomal
abnormalities.
Fig. 1 Chromosomes during cell division. A, Example of photomicrograph. B,
Chromosomes arranged in karyotype; female and male
sex-determining chromosomes.
Chromosomal abnormalities
Errors
resulting in chromosomal abnormalities can occur in mitosis or meiosis. These
occur in either the autosomes or the sex chromosomes.
Even without the presence of obvious structural malformations, small deviations
in chromosomes can cause problems in fetal development.
Autosomal
abnormalities involve differences in the number or structure of chromosomes
resulting from unequal distribution of the genetic material during gamete formation.
Abnormalities of chromosome number, or aneuploidy, are
most often caused by nondisjunction. Nondisjunction occurs during meiosis when a pair of
chromosomes fails to separate, and one resulting cell contains both chromosomes
while the other contains none. The product of the union of a normal gamete with
a gamete containing an extra chromosome is a trisomy.
The resulting individual has 47 chromosomes in each cell. The most common trisomal abnormality is Down syndrome, or trisomy 21.
The
product of the union of a normal gamete
(ovum or sperm) with a gamete missing a chromosome is a monosomy. This individual would
have only 45 chromosomes in each cell. Missing an autosomal
chromosome always results in death of the embryo.
Nondisjunction can also
occur during mitosis. If this occurs early in development when cell lines are
forming, the individual has a mixture of cells, some with a normal number of
chromosomes and others either missing a chromosome or containing an extra
chromosome. This condition is known as mosaicism.
Abnormalities
of chromosome structure involve chromosome breakage, usually resulting from one
of two events: (1) translocation and (2) additions or deletions (or both).
Translocation occurs when genetic material is transferred from one chromosome
to another, different chromosome. Thus instead of two normal pairs of
chromosomes, the individual has one normal chromosome of each pair and a third
chromosome that is a fusion of the other two chromosomes. As long as all
genetic material is retained in the cell, the individual is unaffected but is a
carrier of a balanced translocation.
If
a gamete receives the two normal chromosomes or the fused chromosome, the
resulting offspring will be clinically normal. If the gamete receives one of
the two normal chromosomes and the fused version, the resulting offspring will
have an extra copy of one of the chromosomes. This condition is termed an
unbalanced translocation and often has serious clinical effects.
Whenever
a portion of a chromosome is deleted from one chromosome and added to another,
the gamete produced may have either extra copies of genes or too few copies.
The clinical effects produced may be mild or severe depending on the amount of
genetic material involved.
Several
sex chromosome abnormalities have been identified that are caused by nondisj unction during gametogenesis
in either parent. The most common deviation in females is Turner syndrome, or monosomy X (having only one X chromosome); the affected
female exhibits juvenile external genitalia with undeveloped ovaries. She is
usually short in stature with webbing of the neck. Intelligence may be
impaired. Most affected embryos miscarry spontaneously.
The
most common deviation in males is Klinefelter syndrome,
or trisomy of the sex chromosomes XXY (an extra X
chromosome). The affected male has poorly developed secondary sexual
characteristics and small testes. He is infertile, usually tall, and
effeminate. Males who are mosaic for Klinefelter
syndrome may be fertile. Subnormal intelligence is usually present.
PATTERNS OF GENETIC
TRANSMISSION
Heritable
characteristics are those that can be passed on to offspring. The patterns by
which genetic material is transmitted to the next generation are affected by
the number of genes involved in the expression of the trait. Many phenotypic
characteristics result from two or more genes on different chromosomes acting
together (multifactorial inheritance); others are controlled by
a single gene (unifactorial inheritance).
Defects
at the gene level cannot be determined by conventional laboratory methods such
as karyotyping. Instead, genetic counselors predict
the probability of the presence of an abnormal gene from the known occurrence
of the trait in the individual's family and the known patterns by which the
trait is inherited.
Most
common congenital malformations, such as cleft lip and palate and neural tube
defects, result from multifactorial inheritance, a
combination of genetic and environmental factors. Each malformation may range
from mild to severe, depending on the number of genes for the defect present or
the amount of environmental influence. Multifactorial
disorders tend to occur in families. Some malformations occur more often in one
sex than the other.
If
a single gene controls a particular trait, disorder, or defect, its pattern of
inheritance is referred to as unifactorial mendelian, or single-gene,
inheritance. The number of unifactorial abnormalities
far exceeds the number of chromosomal abnormalities. This is understandable
considering that 50,000 to 100,000 genes in the haploid number (23) of
chromosomes are passed on to an offspring from each parent.
Unifactorial or
single-gene disorders follow the inheritance patterns of dominance,
segregation, and independent assortment described by Mendel
and include autosomal dominant, autosomal recessive, and X-linked dominant and recessive
modes of inheritance (Fig. 2).
Fig. 2 Possible offspring in three types of matings. A,
Homozygous-dominant parent and homozygous-recessive
parent. Children all heterozygous, displaying dominant
trait. B, Heterozygous parent and homozygous-recessive parent. Children
50% heterozygous, displaying dominant
trait; 50% homozygous, displaying recessive trait. C,
Both parents heterozygous. Children 25% homozygous,
displaying dominant trait; 25% homozygous, displaying recessive trait; 50% heterozygous,
displaying dominant trait.
Autosomal dominant
inheritance disorders are those in which the abnormal gene for the trait is
expressed even when the other member of the pair is normal. The abnormal gene
may appear as a result of a mutation, a spontaneous and permanent change in the
normal gene structure. In this case the disorder occurs for the first time in
the family. Usually an affected individual comes from multiple generations
having the disorder (Fig. 2, B and Q. Males and females are equally affected.
Examples
of common autosomal dominantly inherited disorders
are Marfan syndrome (a disorder of connective tissue
resulting in skeletal, ocular, and cardiovascular abnormalities),achondroplasia (dwarfism), polydactyly (extra digits),
Autosomal recessive
inheritance disorders are those in which both genes of a pair must be abnormal
for the disorder to be expressed. Heterozygous individuals have only one
abnormal gene and are unaffected clinically because their normal gene
overshadows the abnormal gene. They are known as carriers of the recessive
trait. For the trait to be expressed, two carriers must each contribute the
abnormal gene to the offspring (see Fig. 2, Q. Males and females are equally
affected. Most inborn errors of metabolism, such as phenylketonuria
(PKU), galactosemia, maple syrup urine disease, Tay-Sachs disease, sickle cell anemia, and cystic fibrosis,
are autosomal recessive inherited disorders (see
Table 19-3 for screening tests for inborn errors of metabolism).
X-linked
dominant inheritance disorders occur in males and heterozygous females. Because
the females also have a normal gene, the effects are less severe than in
affected males. Affected males transmit the abnormal gene only to their
daughters on the X chromosome. Fragile X syndrome is an example of an X-linked
dominant inherited disorder (see Plan of Care).
Abnormal
genes for X-linked recessive inheritance disorders are carried on the X
chromosome. Females may be heterozygous or homozygous for traits carried on the
X chromosome because they have two X chromosomes. Males are hemizygous
because they have only one X chromosome carrying genes, with no alleles on the
Y chromosome. Therefore X-linked recessive disorders are most often manifested
in the male with the abnormal gene on his single X chromosome. Hemophilia,
color blindness, and Duchenne muscular dystrophy are
all X-linked recessive disorders.
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PLAN OF CARE The Family with a Neonate
with Fragile X Syndrome NURSING DIAGNOSIS Risk for
interrupted family processes related to birth of a neonate with an inherited
disorder Expected Outcome The couple will verbalize accurate information about fragile X
disorder, including implications for future pregnancies. Nursing Interventions Rationales Assess
knowledge base of couple regarding the clinical signs and symptoms of fragile
X syndrome and inheritance patterns to correct any misconceptions and
establish basis for teaching plan. Provide
information throughout the genetics evaluation regarding risk status and
clinical signs and symptoms of fragile X syndrome to give couple a realistic
picture of neonate's defects and assist with decision making for future pregnancies. Use
therapeutic communication during discussions with the couple to provide
opportunity for expression of concern. Refer
to support groups, social services, or counseling to assist with family
cohesive actions and decision making. Refer
to child development specialist to provide family with realistic expectations
regarding cognitive and behavioral differences of child with fragile X
syndrome. NURSING DIAGNOSIS
Situational low self-esteem related to diagnosis of inherited disorder as
evidenced by parents" statements of guilt and shame Expected Outcome
The parents will express an increased number of positive statements regarding
the birth of a neonate with fragile X syndrome. Nursing Interventions Rationales Assist
parents to list strengths and coping strategies that have been helpful in
past situations to use appropriate strategies during this situational crisis.
Encourage
expression of feelings using therapeutic communication to provide
clarification and emotional support. Clarify
and provide information regarding fragile X syndrome to decrease feelings of
guilt and gradually increase feelings of positive self-esteem. Refer
for further counseling as needed to provide more indepth
and ongoing support. NURSING DIAGNOSIS
Risk for impaired parenting related to birth of neonate with fragile X syndrome Nursing Interventions Rationales Assist
parents to see and describe normal aspects of infant to promote bonding. Encourage
and assist with breastfeeding if that is parent's choice of feeding method to
facilitate closeness with infant and provide benefits of breastmilk. Assure
parents that information regarding the neonate will remain confidential to
assist the parents to maintain some situational control and allow for time to
work through their feelings. Discuss
and role play with parents ways of informing family and friends of infant's
diagnosis and prognosis to promote positive aspects of infant and decrease
potential isolation from social interactions. Provide
anticipatory guidance about what to expect as infant develops to assist
family to be prepared for behavior problems or mental deficits. NURSING DIAGNOSIS
Spiritual distress related to situational crisis of child born with fragile X
syndrome Expected Outcome
Parents seek appropriate support persons (family members, priest, minister, rabbi)
for assistance. Nursing Interventions Rationales Listen
for cues indicative of parent's feelings ("Why did God do this to
us?") to identify messages indicating spiritual distress. Acknowledge
parents' spiritual concerns and encourage expression of feelings to help
build a therapeutic relationship. Facilitate
visits from clergy and provide privacy during visits to demonstrate respect
for parent's relationship with clergy. Encourage
parents to discuss concerns with clergy to use expert spiritual care
resources to help the parents. Facilitate
interaction with family members and other support persons to encourage
expressions of concern and seek comfort. |
NONGENETIC FACTORS
INFLUENCING DEVELOPMENT
Not
all congenital disorders are inherited. Congenitalmeans
that the condition was present at birth. Some congenital malformations may be
the result of teratogens, that is, environmental
substances or exposures that result in functional or structural disability. In
contrast to other forms of developmental disabilities, disabilities caused by teratogens are, in theory, totally preventable. Known human
teratogens are drugs and chemicals, infections,
exposure to radiation (Scialli, 1997), and certain
maternal conditions such as diabetes and PKU (
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ENVIRONMENTAL Maternal Conditions Alcoholism, diabetes, endocrinopathies, phenylketonuria, smoking, nutritional problems Infectious Agents Rubella, toxoplasmosis, syphilis, herpes simplex, cytomegalic inclusion disease, varicella, Venezuelan equine encephalitis Mechanical Problems
(Deformations) Amniotic band constrictions, umbilical cord constraint, disparity in
uterine size and uterine contents Chemicals, Drugs, Radiation,
Hyperthermia GENETIC Single Gene Disorders Chromosomal Abnormalities UNKNOWN Polygenic/Multifactorial (Gene-Environment Interactions) "Spontaneous"
Errors of Development Other Unknowns Modified from Fanaroff,
A., & Martin, R. (1997). Neonatal-perinatal
medicine: Diseases of the fetus and infant. |
Fig. 3 Sensitive, or critical, periods in human development.
Dark color denotes highly sensitive periods; light color indicates stages that
are less sensitive to teratogens. (From
Moore, K., & Persaud, T. [1998]. Before we
are born: Essentials of embryology and birth defects [5th ed.].
In
addition to genetic makeup and the influence of teratogens,
the adequacy of maternal nutrition influences development. The embryo and fetus
must obtain the nutrients they need from the mother's diet; they cannot tap the
maternal reserves. Malnutrition during pregnancy produces low-birth-weight
newborns who are susceptible to infection.
Malnutrition also affects brain development during the latter half of gestation
and may result in learning disabilities in the child.
The
field of behavioral genetics is engaged in discovering links between genetics
and environment in explaining normal and deviant behavior (Sherman et al.,
1997). This represents a movement away from the belief that human behavior is
almost completely the result of influences of the environment. For example,
memory and intelligence, activity level, sociability, and shyness have some
degree of genetic influence (Sherman et al., 1997).
CONCEPTION
CELL DIVISION
Cells
are reproduced by two different methods: mitosis and meiosis. In mitosis, the
body cells replicate to yield two cells with the same genetic makeup as the
parent cell. First the cell makes a copy of its DNA; then it divides, and each
daughter cell receives one copy of the genetic material. The purpose of mitotic
division is for growth and development or cell replacement.
Meiosis
produces gametes (eggs and sperm). Each homologous pair of chromosomes contains
one chromosome received from the mother and one from the father; thus meiosis
results in cells that contain one of each of the 23 pairs of chromosomes.
Because these germ cells contain 23 single chromosomes, half of the genetic
material of a normal somatic cell, they are termed haploid. When the female gamete
(egg or ovum) and the male gamete (spermatozoon) unite to form the zygote, the
diploid number of human chromosomes (46, or 23 pairs) is restored.
The
process of DNA replication and cell division in meiosis allows different
alleles for genes to be distributed at random by each parent and then
rearranged on the paired chromosomes. The chromosomes then separate and proceed
to different gametes. Because the two parents have genotypes derived from four
different grandparents, many combinations of genes on each chromosome are
possible. This random mixing of alleles accounts for the variation of traits
seen in the children of the same two parents.
GAMETOGENESIS
When
a male reaches puberty, his testes begin the process of spermatogenesis. The
cells that undergo meiosis in the male are termed spermatocytes.
The primary spermatocyte, which undergoes the first
meiotic division, contains the diploid number of chromosomes. The cell has
already copied its DNA before division, so four alleles for each gene are
present. Because the copies are bound together (i.e., one allele plus its copy on
each chromosome), the cell is still considered diploid.
During
the first meiotic division, two haploid secondary spermatocytes
are formed, each containing 22 autosomes and one sex
chromosome; one contains the X chromosome (plus its copy) and the other the Y
chromosome (plus its copy). During the second meiotic division the male
produces two gametes with an X chromosome and two gametes with a Y chromosome,
all of which will develop into viable sperm (Fig. 4, A).
Fig 4 A, Spermatogenesis. Gametogenesis in the male produces four mature gametes, the
sperm. B, Oogenesis. Gametogenesis
in the female produces one mature ovum and three polar bodies. Note relative
difference in overall size between ovum and sperm. C, Fertilization results in
the single-cell zygote and restoration of the diploid number of chromosomes.
Oogenesis,
the process of egg (ovum) formation, begins during fetal life of the female.
All the cells that may undergo meiosis in a woman's lifetime are contained in
her ovaries at birth. The majority of the estimated 2 million primary oocytes (the cells that undergo the first meiotic division)
degenerate spontaneously. Only 400 to 500 ova will mature during the
approximately 35 years of a woman's reproductive life. The primary oocytes begin the first meiotic division (i.e., they
replicate their DNA) during fetal life but remain suspended at this stage until
puberty (Fig. 4, B). Then, usually monthly, one primary oocyte
matures and completes the first meiotic division, yielding two unequal cells:
the secondary oocyte and a small polar body. Both contain
22 autosomes and one X sex chromosome.
At
ovulation the second meiotic division begins. However, the ovum does not
complete the second meiotic division unless fertilization occurs. At
fertilization, a second polar body and the zygote (the united egg and sperm)
are produced (Fig. 4, Q. The three polar bodies degenerate. If fertilization
does not occur, the ovum also degenerates.
Conception
Conception,
defined as the union of a single egg and sperm, marks the beginning of a
pregnancy. Conception does not occur in isolation; a number of events surround it.
These events include gamete (egg and sperm) formation, ovulation (release of
the egg), union of the gametes (which results in an embryo), and implantation
in the uterus.
Ovum.
Meiosis, the process by which germ cells divide and decrease
their chromosomal number by half, occurs in the female in the ovarian
follicles and produces an egg, or ovum. Each month, one ovum matures with a
host of surrounding supportive cells.
At
ovulation the ovum is released from the ruptured ovarian follicle. High
estrogen levels increase the motility of the uterine tubes so their cilia are
able to capture the ovum and propel it through the tube toward the uterine cavity.
An ovum cannot move by itself.
Two
protective layers surround the ovum (Fig. 5). The inner layer is a thick, acellular layer, the zona
pelluada. The outer layer, the corona radiata, is composed of elongated cells.
Fig. 5 Sperm and ovum.
Ova
are considered fertile for approximately 24 hours after ovulation. If
unfertilized by a sperm, the ovum degenerates and is reabsorbed.
Sperm. Ejaculation
during sexual intercourse normally propels almost a teaspoon of semen
containing as many as 200 million to 500 million sperm into the vagina. The sperm
swim with the flagellar movement of their tails. Some
sperm can reach the site of fertilization within 5 minutes, but average transit
time is 4 to 6 hours. Sperm remain viable within the woman's reproductive
system for an average of 2 to 3 days. Most sperm are lost in the vagina, within
the cervical mucus, or in the endometrium, or they
enter the tube that contains no ovum.
As
sperm travel through the female reproductive tract, enzymes are produced to aid
in their capacitation. Capacitation
is a physiologic change that removes the protective coating from the heads
of the sperm. Small perforations then form in the acrosome
(a cap on the sperm) and allow enzymes (e.g., hyaluronidase)
to escape. These enzymes are necessary for the sperm to penetrate the
protective layers of the ovum before fertilization.
FERTILIZATION
Fertilization
takes place in the ampulla
(outer third) of the uterine tube. When a sperm successfully penetrates the membrane
surrounding the ovum, both sperm and ovum are enclosed within the membrane, and
the membrane becomes impenetrable to other sperm; this is termed the zona reaction. The second meiotic division of
the oocyte is then completed, and the ovum nucleus
becomes the female pronucleus. The head of the sperm
enlarges to become the male pronucleus, and the tail
degenerates. The nuclei fuse and the chromosomes combine, restoring the diploid
number (46) (Fig. 6). Conception, the formation of the zygote, has been
achieved.
Fig. 6 Fertilization. A, Ovum
fertilized by X-bearing sperm to form female zygote. B, Ovum fertilized by
Y-bearing sperm to form male zygote.
Mitotic
cellular replication, called cleavage, begins as the zygote travels the
length of the uterine tube into the uterus. This voyage takes 3 to 4 days.
Because the fertilized egg divides rapidly with no increase in size,
successively smaller cells, blastomeres, are
formed with each division. A 16-cell morula,
a solid ball of cells, is produced within 3 days, and is still surrounded
by the protective zona pellucida
(Fig. 7, A). Further development occurs as the morula
floats freely within the uterus. Fluid passes through the zona
pellucida into the intercellular spaces between the blastomeres, separating them into two parts: the trophoblast (which gives rise to the placenta) and the embryoblast (which gives rise to the embryo). A cavity
forms within the cell mass as the spaces come together, forming a structure
termed the blastocyst cavity. When the
cavity becomes recognizable, the whole structure of the developing embryo is
known as the blastocyst. The outer
layer of cells surrounding the cavity is the trophoblast.
Fig. 7, First week of human development. A, Follicular development in the ovary, ovulation, fertilization,
and transport of the early embryo down the uterine tube and into the uterus,
where implantation occurs. B, Blastocyst
embedded in endometrium. Germ
layers forming. (A, From Carlson, B. [1994]. Human embryology and developmental biology.
IMPLANTATION
The
zona pellucida degenerates,
and the trophoblast attaches itself to the uterine endometrium, usually in the anterior or posterior fundal region. Between 6 and 10 days after conception, the trophoblast secretes enzymes that enable it to burrow into
the endometrium until the entire blastocyst
is covered. This is termed implantation. Endometrial blood vessels
erode, and some women experience implantation bleeding (slight spotting and
bleeding during the time of the first missed menstrual period). Chorionic villi, or
fingerlike projections, develop out of the trophoblast
and extend into the blood-filled spaces of the endometrium.
These villi are vascular processes that obtain oxygen
and nutrients from the maternal bloodstream and dispose of carbon dioxide and
waste products into the maternal blood.
After
implantation, the endometrium is termed the decidua. The portion directly under the blastocyst, where the chorionic villi tap the maternal blood vessels, is the deciduas basalis. The portion covering the blastocyst
is the decidua capsularis,
and the portion lining the rest of the uterus is the decidua
vera (Fig. 8).
Fig. 8 Development of fetal membranes.
Note gradual obliteration of intrauterine cavity as decidua
capsularis and decidua vera meet. Also note thinning of
uterine wall. Chorionic and amniotic membranes are in
opposition to each other but may be peeled apart.
EMBRYO AND FETUS
Pregnancy
lasts approximately 10 lunar months (9 calendar months, 40 weeks, or 280 days).
Length of pregnancy is computed from the first day of the last menstrual period
(LMP) until the day of birth. However, conception occurs approximately 2 weeks
after the first day of the LMP. Thus the postconception
age of the fetus is 2 weeks less, for a total of 266 days, or 38 weeks. Postconception age is used in the discussion of fetal
development.
Intrauterine
development is divided into three stages: ovum or preembryonic,
embryo, and fetus (see Fig. 3). The stage of the ovum lasts from conception
until day 14. This period covers cellular replication, blastocyst
formation, initial development of the embryonic membranes, and establishment of
the primary germ layers.
PRIMARY GERM LAYERS
During
the third week after conception the embryonic disk differentiates into three
primary germ layers: the ectoderm, mesoderm, and endoderm or entoderm (Fig. 7, B). All tissues and organs of the embryo
develop from these three layers.
The
ectoderm, or upper layer of the
embryonic disk, gives rise to the epidermis, glands, nails and hair, central and
peripheral nervous systems, lens of the eye, tooth enamel, and floor of the
amniotic cavity.
The
mesoderm, or middle layer, develops
into the bones and teeth, muscles (skeletal, smooth, and cardiac), dermis and
connective tissue, cardiovascular system and spleen, and urogenital
system.
The
endoderm, or lower layer, gives rise
to the epithelium lining the respiratory tract and digestive tract, including
the oropharynx, liver and pancreas, urethra, bladder,
and vagina. The endoderm forms the roof of the yolk sac.
DEVELOPMENT OF THE
EMBRYO
The
stage of the embryo lasts from day 15 until approximately 8 weeks after
conception, or until the embryo measures 3 cm from crown to rump. The embryonic
stage is the most critical time in the development of the organ systems and the
main external features. Developing areas with rapid cell division are the most
vulnerable to malformation by environmental teratogens.
At the end of the eighth week, all organ systems and external structures are
present, and the embryo is unmistakably human (see Fig. 3).
Membranes
At
the time of implantation, two fetal membranes that will surround the developing
embryo begin to form. The chorion develops from the trophoblast and contains the chorionic
villi on its surface. The villi
burrow into the deciduas basalis and increase in size
and complexity as the vascular processes develop into the placenta. The chorion becomes the covering of the fetal side of the
placenta. It contains the major umbilical blood vessels that branch out over
the surface of the placenta. As the embryo grows, the decidua
capsularis stretches. The chorionic
villi on this side atrophy and degenerate, leaving a
smooth chorionic membrane.
The
inner cell membrane, the amnion, develops from the interior cells of the blastocyst. The cavity that develops between this inner
cell mass and the outer layer of cells (trophoblast)
is the amniotic cavity (see Fig. 7, B). As it grows larger, the amnion forms on
the side opposite to the developing blastocyst (see
Figs. 7, B, and 8). The developing embryo draws the amnion around itself to
form a fluid-filled sac. The amnion becomes the covering of the umbilical cord
and covers the chorion on the fetal surface of the
placenta. As the embryo grows larger, the amnion enlarges to accommodate the
embryo/fetus and the surrounding amniotic fluid. The amnion eventually comes in
contact with the chorion surrounding the fetus.
Amniotic fluid
At
first the amniotic cavity derives its fluid by diffusion from the maternal
blood. The amount of fluid increases weekly, and 800
to 1200 ml of transparent liquid is normally present at term. The amniotic
fluid volume changes constantly. The fetus swallows fluid, and fluid flows into
and out of the fetal lungs. The fetus urinates into the fluid, greatly
increasing its volume.
Many
functions are served by amniotic fluid for the embryo/fetus. Amniotic fluid
helps maintain a constant body temperature. It serves as a source of oral fluid
and as a repository for waste. It cushions the fetus from trauma by blunting
and dispersing outside forces. It allows freedom of movement for
musculoskeletal development. The fluid keeps the embryo from tangling with the
membranes, facilitating symmetric growth of the fetus. If the embryo does
become tangled with the membranes, amputations of extremities or other
deformities can occur from constricting amniotic bands.
The
volume of amniotic fluid is an important factor in assessing fetal well-being.
Having less than 300 ml of amniotic fluid (oligohydramnios)
is associated with fetal renal abnormalities. Having more than 2 L of
amniotic fluid (hydramnios) is associated with
gastrointestinal and other malformations.
Amniotic
fluid contains albumin, urea, uric acid, creatinine, lecithin,
sphingomyelin, bilirubin,
fructose, fat, leukocytes, proteins, epithelial cells, enzymes, and lanugo hair. Study of fetal cells in amniotic fluid through
amniocentesis yields much information about the fetus. Genetic studies (karyotyping) provide knowledge about the sex and the number
and structure of chromosomes. Other studies, such as the lecithin/sphingomyelin ratio, determine the health or maturity of
the fetus.
Yolk sac
At
the same time the amniotic cavity and amnion are forming, another blastocyst cavity forms on the other side of the developing
embryonic disk (see Fig. 7, B). This cavity becomes surrounded by a membrane,
forming the yolk sac. The yolk sac aids in transferring maternal nutrients and oxygen,
which have diffused through the chorion, to the embryo.
Blood vessels form to aid transport. Blood cells and plasma are manufactured in
the yolk sac during the second and third weeks. At the end of the third week,
the primitive heart begins to beat and circulate the blood through the embryo,
connecting stalk, chorion, and yolk sac.
The
folding in of the embryo during the fourth week results in incorporation of
part of the yolk sac into the embryo's body as the primitive digestive system.
Primordial germ cells arise in the yolk sac and move into the embryo. The
shrinking remains of the yolk sac degenerate (see Fig. 7, B). By the fifth or
sixth week, the remnant has separated from the embryo.
Umbilical cord
By
day 14 after conception the embryonic disk, amniotic sac, and yolk sac are
attached to the chorionic villi
by the connecting stalk. During the third week the blood vessels develop to
supply the embryo with maternal nutrients and oxygen. During the fifth week,
after the embryo has curved inward on itself from both ends (bringing the
connecting stalk to the ventral side of the embryo), the connecting stalk
becomes compressed from both sides by the amnion, forming the narrower
umbilical cord (see Fig. 8). Two arteries carry blood to the chorionic villi from the embryo,
and one vein returns blood to the embryo. Approximately 1% of umbilical cords
contain only two vessels: one artery and one vein. This occurrence is sometimes
associated with congenital malformations.
The
cord rapidly increases in length. At term the cord is 2 cm in diameter and
ranges from 30 to 90 cm in length (with an average of 55 cm). It twists
spirally on itself and loops around the embryo/fetus. A true knot is rare, but
false knots occur as folds or kinks in the cord and may jeopardize circulation
to the fetus. Connective tissue called Wharton's jelly prevents
compression of the blood vessels and ensures continued nourishment of the
embryo/fetus. Compression can occur if the cord lies between the fetal head and
the pelvis or if it is twisted around the fetal body. When the cord is wrapped
around the fetal neck, it is termed a nuchal
cord.
Because
the placenta develops from the chorionic villi, the umbilical cord is usually located centrally. A
peripheral location is less common and is termed battledore placenta. The
blood vessels are arrayed out from the center to all parts of the placenta.
PLACENTA
Structure. The
placenta begins to form at implantation. During the third week after
conception, the trophoblast cells of the chorionic villi continue to
invade the decidua basalis.
As the uterine capillaries are tapped, the endometrial spiral arteries fill
with maternal blood. The chorionic villi grow into the spaces with two layers of cells: the
outer syncytium and the inner cytotrophoblast.
A third layer develops into anchoring septa, dividing the projecting decidua into separate areas called cotyledons. In
each of the 15 to 20 cotyledons, the chorionic villi branch out, and a complex system of fetal blood
vessels forms. Each cotyledon is a functional unit. The whole structure is the
placenta (Fig. 9).
Fig. 9 Term placentas. A, Maternal (or uterine) surface, showing cotyledons and grooves.
B, Fetal (or amniotic) surface, showing blood vessels running
under amnion and converging to form umbilical vessels at attachment of
umbilical cord. C, Amnion and smooth chorion
are arranged to show that they are (1) fused and (2) continuous with margins of
placenta. (Courtesy Marjorie Pyle, RNC, Lifecircle,
The
maternal-placental-embryonic circulation is in place by day 17, when the
embryonic heart starts beating. By the end of the third week, embryonic blood
is circulating between the embryo and the chorionic villi. In the intervillous spaces,
maternal blood supplies oxygen and nutrients to the embryonic capillaries in
the villi (Fig. 10). Waste products and carbon
dioxide diffuse into the maternal
blood.
Fig . 1 0
Schematic drawing of the placenta illustrating how it supplies oxygen and
nutrition to the embryo and removes its waste products. Deoxygenated
blood leaves the fetus through the umbilical arteries and enters the placenta, where
it is oxygenated. Oxygenated blood leaves the placenta through the umbilical
vein, which enters the fetus via the umbilical cord. (From
Moore, K., & Persaud, T. [1998]. Before we
are born: Essentials of embryology and birth defects [5th ed.[.
The
maternal-placental-embryonic circulation is in place by day 17, when the
embryonic heart starts beating. By the end of the third week, embryonic blood
is circulating between the embryo and the chorionic villi. In the intervillous spaces,
maternal blood supplies oxygen and nutrients to the embryonic capillaries in
the villi (Fig. 7-10). Waste products and carbon
dioxide diffuse into the maternal blood.
The
placenta functions as a means of metabolic exchange. Exchange is minimal at
this time because the two cell layers of the villous membrane are too thick.
Permeability increases as the cytotrophoblast thins
and disappears; by the fifth month, only the single layer of syncytium is left between the maternal blood and the fetal capillaries.
The syncytium is the functional layer of the placenta.
By the eighth week, genetic testing may be done on a sample of chorionic villi obtained by
aspiration biopsy; however, limb defects have been associated with chorionic villi sampling done
before 10 weeks. The structure of the placenta is complete by the twelfth week.
The placenta continues to grow wider until 20 weeks, when it covers approximately
half of the uterine surface. It then continues to grow thicker. The branching villi continue to develop within the body of the placenta,
increasing the functional surface area.
Functions.
One of the early functions of the placenta is as an endocrine gland that
produces four hormones necessary to maintain the pregnancy and support the
embryo/fetus. The hormones are produced in the syncytium.
The
protein hormone human chorionic gonadotropin
(hCG) can be detected in the
maternal serum by 8 to 10 days after conception, or shortly after implantation.
This hormone is the basis for pregnancy tests. The hCG preserves the function of the ovarian corpus luteum, ensuring a continued supply of estrogen and
progesterone needed to maintain the pregnancy. Miscarriage occurs if the corpus
luteum stops functioning before the placenta is
producing sufficient estrogen and progesterone. The hCG reaches its maximum level at 50 to 70 days, then
begins to decrease.
The
other protein hormone produced by the placenta is chorionic
somatomammotropin, or human placental lactogen. This substance is similar to a growth hormone and
stimulates maternal metabolism to supply needed nutrients for fetal growth.
This hormone increases the resistance to insulin, facilitates glucose transport
across the placental membrane, and stimulates breast development to prepare for
lactation.
The
placenta eventually produces more of the steroid hormone progesterone than the
corpus luteum does during the first few months of
pregnancy. Progesterone maintains the endometrium,
decreases the contractility of the uterus, and stimulates development of breast
alveoli and maternal metabolism.
By
7 weeks after fertilization, the placenta is producing most of the maternal
estrogens, which are steroid hormones. The major estrogen secreted by the
placenta is estriol, and the ovaries produce mostly estradiol. Measuring estriol
levels is a clinical assay for placental functioning. Estrogen stimulates uterine
growth and uteroplacental blood flow. It causes a
proliferation of the breast glandular tissue and stimulates myometrial
contractility. Placental estrogen production increases greatly toward the end
of pregnancy. One theory for the cause of the onset of labor is the decrease in
circulating levels of progesterone and the increased levels of estrogen.
The
metabolic functions of the placenta are respiration, nutrition, excretion, and
storage. Oxygen diffuses from the maternal blood across the placental membrane
into the fetal blood, and carbon dioxide diffuses in the opposite direction. In
this way the placenta functions as a lung for the fetus.
Carbohydrates,
proteins, calcium, and iron are stored in the placenta for ready access to meet
fetal needs. Water, inorganic salts, carbohydrates, proteins, fats, and
vitamins pass from the maternal blood supply across the placental membrane into
the fetal blood, supplying nutrition. Water and most electrolytes with a
molecular weight less than 500 readily diffuse through the membrane.
Hydrostatic and osmotic pressures aid in the flow of water and some solutions.
Facilitated and active transport assist in the transfer
of glucose, amino acids, calcium, iron, and substances with higher molecular
weights. Amino acids and calcium are transported against the concentration
gradient between the maternal blood and fetal blood.
The
fetal concentration of glucose is lower than the glucose level in the maternal
blood because of its rapid metabolism by the fetus. This fetal requirement
demands larger concentrations of glucose than simple diffusion can provide.
Therefore maternal glucose moves into the fetal circulation by active
transport.
Pinocytosis is a
mechanism used for transferring large molecules (e.g., albumin and gamma
globulins) across the placental membrane. This mechanism conveys the maternal immunoglobulins that provide early passive immunity to the
fetus.
Metabolic
waste products of the fetus cross the placental membrane from the fetal blood
into the maternal blood. The maternal kidneys then excrete them. Many viruses
can cross the placental membrane and infect the fetus. Some bacteria and
protozoa first infect the placenta and then infect the fetus. Drugs can also
cross the placental membrane and may harm the fetus. Caffeine, alcohol, nicotine,
carbon monoxide and other toxic substances in cigarette smoke, and prescription
and recreational drugs (e.g., marijuana and cocaine) readily cross the
placenta.
Although
no direct link exists between the fetal blood in the vessels of the chorionic villi and the maternal
blood in the intervillous spaces, only one cell layer
separates them. Breaks occasionally occur in the placental membrane. Fetal erythrocytes
then leak into the maternal circulation, and the mother may develop antibodies
to the fetal red blood cells. This is often how the Rh-negative mother becomes sensitized
to the erythrocytes of her Rh-positive fetus.
Although
the placenta and fetus are living tissue transplants, they are not destroyed by
the host mother (Cunningham et al., 2001). Either the placental hormones suppress
the immunologic response, or the tissue evokes no response.
Placental
function depends on the maternal blood pressure supplying the circulation.
Maternal arterial blood, under pressure in the small uterine spiral arteries,
spurts into the intervillous spaces (see Fig. 10). As
long as rich arterial blood continues to be supplied, pressure is exerted on
the blood already in the intervillous spaces, pushing
it toward drainage by the low-pressure uterine veins. At term gestation, 10% of
the maternal cardiac output goes to the uterus.
If
there is interference with the circulation to the placenta, the placenta cannot
supply the embryo/fetus. Vasoconstriction, such as that caused by hypertension
and cocaine use, diminishes uterine blood flow. Decreased maternal blood pressure
or cardiac output also diminishes uterine blood flow.
When
a woman lies on her back with the pressure of the uterus compressing the vena
cava, blood return to the right atrium is diminished. Excessive maternal
exercise that diverts blood to the muscles away from the uterus compromises placental
circulation. Optimal circulation is achieved when the woman is lying at rest on
her side. Decreased uterine circulation may lead to intrauterine growth
restriction of the fetus and infants who are small for gestational age.
Braxton
Hicks contractions seem to enhance the movement of blood through the intervillous spaces, aiding placental circulation. However,
prolonged contractions or too-short intervals between contractions during labor
reduce blood flow to the placenta.
FETAL MATURATION
The
stage of the fetus lasts from approximately 9 weeks (when the embryo becomes
recognizable as a human being) until the pregnancy ends. Changes during the
fetal period are not as dramatic, because refinement of structure and function
are taking place. The fetus is less vulnerable to teratogens
except for those affecting central nervous system functioning. Viability refers
to the capability of the fetus to survive outside the uterus. In the past the
earliest age at which fetal survival could be expected was 28 weeks after
conception. With modern technology and advances in maternal and neonatal care,
viability is now possible at 20 weeks after conception (22 weeks since LMP;
fetal weight of 500 g or more). The limitations on survival outside the uterus
are based on central nervous system function and oxygenation capability of the
lungs.
Respiratory system
The
respiratory system begins development during embryonic life and continues
through fetal life and into childhood. The development of the respiratory tract
begins in week 4 and continues through week 17 with formation of the trachea,
bronchi, and lung buds. Between 16 and 24 weeks, the bronchi and terminal
bronchioles enlarge and vascular structures and primitive alveoli are formed.
Between 24 weeks and term birth, more alveoli form. Specialized alveolar cells,
type I and type II cells, secrete pulmonary
surfactants to line the interior of the alveoli. After 32 weeks, sufficient surfactant
is present in developed alveoli to provide infants with a good chance of
survival.
Pulmonary
surfactants. The detection of pulmonary surfactants
(surface-active phospholipids) in amniotic fluid has been used to determine the
degree of fetal lung maturity, or the ability of the lungs to function after
birth. Lecithin (L) is the most critical alveolar surfactant required for
postnatal lung expansion. It is detectable at approximately 21 weeks and
increases in amount after the twentyfourth week.
Another pulmonary phospholipid, sphingomyelin
(S), remains constant in amount. Thus the measure of lecithin in relation to sphingomyelin, or the L/S ratio, is used to determine fetal
lung maturity. When the L/S ratio reaches 2:1, the infant's lungs are
considered to be mature. This occurs at approximately 35 weeks of gestation
(Creasy & Resnik, 1999).
Certain
maternal conditions such as maternal hypertension, placental dysfunction,
infection, or corticosteroid use cause decreased maternal placental blood flow
and accelerate lung maturity. This apparently is caused by the resulting fetal
hypoxia, which stresses the fetus and increases the blood levels of
corticosteroids that accelerate alveolar and surfactant development. Conditions
such as gestational diabetes and chronic glomerulonephritis
can retard fetal lung maturity.
Fetal
respiratory movements have been seen on ultrasound as early as the eleventh
week. These fetal respiratory movements may aid in development of the chest
wall muscles and regulate lung fluid volume. The fetal lungs produce fluid that
expands the air spaces in the lungs. The fluid drains into the amniotic fluid
or is swallowed by the fetus.
Before
birth, secretion of lung fluid decreases. The normal birth process squeezes out
approximately one third of the fluid. Infants of cesarean births do not benefit
from this squeezing process; thus they may have more respiratory difficulty at
birth. The fluid remaining in the lungs at birth is usually reabsorbed into the
infant's bloodstream within 2 hours of birth.
Fetal circulatory system
The
cardiovascular system is the first organ system to function in the developing
human. Blood vessel and blood cell formation begins in the third week and
supplies the embryo with oxygen and nutrients from the mother. By the end of
the third week, the tubular heart begins to beat and the primitive
cardiovascular system links the embryo, connecting stalk, chorion,
and yolk sac. During the fourth and fifth weeks, the heart develops into a fourchambered organ. By the end of the embryonic stage, the
heart is developmentally complete.
The
fetal lungs do not function for respiratory gas exchange, so a special
circulatory pathway, the ductus arteriosus,
bypasses the lungs. Oxygen-rich blood from the placenta flows rapidly through
the umbilical vein into the fetal abdomen (Fig. 11). When the umbilical vein
reaches the liver, it divides into two branches. One branch circulates some
oxygenated blood through the liver. Most of the blood passes through the ductus venosus into the inferior vena
cava. There it mixes with the deoxygenated blood from the fetal legs and
abdomen on its way to the right atrium. Most of this blood passes straight
through the right atrium and through the foramen ovale,
an opening into the left atrium. There it mixes with the small amount of blood
returning deoxygenated from the fetal lungs through the pulmonary veins.
Fig. 7-11 Schematic illustration of the fetal circulation.
The colors indicate the oxygen saturation of the blood, and the arrows show the
course of the blood from the placenta to the heart. The organs are not drawn to
scale. Observe that three shunts permit most of the blood to bypass the liver
and lungs: (1) ductus venosus,
(2) foramen ovale, and (3) ductus
arteriosus. The poorly oxygenated blood returns to
the placenta for oxygen and nutrients through the umbilical arteries. (From Moore, K., & Persaud, T.
[1998]. Before we are born: Essentials of embryology and birth defects [5th
ed.].
The
blood flows into the left ventricle and is squeezed out into the aorta, where
the arteries supplying the heart, head, neck, and arms receive most of the
oxygen-rich blood. This pattern of supplying the highest levels of oxygen and nutrients
to the head, neck, and arms enhances the cephalocaudal
(head-to-rump) development of the embryo/fetus.
Deoxygenated
blood returning from the head and arms enters the right atrium through the
superior vena cava. This blood is directed downward into the right ventricle,
where it is squeezed into the pulmonary artery. A small amount of blood
circulates through the resistant lung tissue, but the majority follows the path
with less resistance through the ductus arteriosus into the aorta, distal to the point of exit of
the arteries supplying the head and arms with oxygenated blood. The oxygen-poor
blood flows through the abdominal aorta into the internal iliac arteries, where
the umbilical arteries direct most of it back through the umbilical cord to the
placenta. There the blood gives up its wastes and carbon dioxide in exchange
for nutrients and oxygen. The blood remaining in the iliac arteries flows through
the fetal abdomen and legs, ultimately returning through the inferior vena cava
to the heart.
Hematopoietic system
Hematopoiesis, or the
formation of blood, occurs in the yolk sac (see Fig. 7, B) beginning in the
third week. Hematopoietic stem cells seed the fetal
liver during the fifth week, and hematopoiesis begins
there during the sixth week. This accounts for the relatively large size of the
liver between the seventh and ninth weeks. Stem cells seed the fetal bone
marrow, spleen and thymus, and lymph nodes between weeks 8 and 11.
The
antigenic factors that determine blood type are present in the erythrocytes
soon after the sixth week. For this reason the Rh-negative woman is at risk for
isoimmunization in any pregnancy that lasts longer
than 6 weeks after fertilization.
Hepatic system
The
liver and biliary tract develop
from the foregut during the fourth week of gestation. Hematopoiesis
begins during the sixth week and requires that the liver be large. The
embryonic liver is prominent, occupying most of the abdominal cavity. Bile, a
constituent of meconium, begins to form in the
twelfth week.
Glycogen
is stored in the fetal liver beginning at week 9 or 10. At term, glycogen
stores are twice those of the adult. Glycogen is the major source of energy for
the fetus and for the neonate stressed by in utero
hypoxia, extrauterine loss of the maternal glucose
supply, the work of breathing, or cold stress.
Iron
is also stored in the fetal liver. If maternal intake is sufficient, the fetus
can store enough iron to last for 5 months after birth.
During
fetal life the liver does not have to conjugate bilirubin
for excretion because the unconjugated bilirubin is cleared by the placenta. Therefore the glucuronyl transferase enzyme
needed for conjugation is present in the fetal liver in amounts less than those
required after birth. This predisposes the neonate to hyperbilirubinemia.
Coagulation
factors II, VII, IX, and X cannot be synthesized in the fetal liver because of
the lack of vitamin K synthesis in the sterile fetal gut. This coagulation
deficiency persists after birth for several days and is the rationale for the
prophylactic administration of vitamin K to the newborn.
Gastrointestinal system
During
the fourth week, the shape of the embryo changes from being almost straight to
a C shape as both ends fold in toward the ventral surface. A portion of the yolk
sac is incorporated into the body from head to tail as the primitive gut
(digestive system).
The
foregut produces the pharynx, part of the lower respiratory tract, the
esophagus, the stomach, the first half of the duodenum, the liver, the
pancreas, and the gallbladder. These structures evolve during the fifth and
sixth weeks. Malformations that can occur in these areas include esophageal atresia, hypertrophic pyloric stenosis, duodenal stenosis or atresia, and biliary atresia.
The
midgut becomes the distal half of the duodenum, the
jejunum and ileum, the cecum and appendix, and the proximal
half of the colon. The midgut loop projects into the
umbilical cord between weeks 5 and 10. A malformation (or omphalocele)
results if the midgut fails to return to the
abdominal cavity, causing the intestines to protrude from the umbilicus. Meckel's diverticulum is the most
common malformation of the midgut. It occurs when a remnant
of the yolk stalk that has failed to degenerate attaches to the ileum, leaving
a blind sac.
The
hindgut develops into the distal half of the colon, the rectum and parts of the
anal canal, the urinary bladder, and the urethra. Anorectal
malformations are the most common abnormalities of the digestive system.
The
fetus swallows amniotic fluid beginning in the fifth month. Gastric emptying
and intestinal peristalsis occur. Fetal nutrition and elimination needs are
taken care of by the placenta. As the fetus nears term, fetal waste products accumulate
in the intestines as dark green to black tarry meconium.
Normally, this substance is passed through the rectum within 48 hours of birth.
Sometimes with a breech presentation or fetal hypoxia, meconium
is passed in utero into the amniotic fluid. The
failure to pass meconium after birth may indicate atresia somewhere in the digestive tract, an imperforate
anus, or meconium ileus, in
which a firm meconium plug blocks passage (seen in
infants with cystic fibrosis).
The
metabolic rate of the fetus is relatively low, but the infant has great growth
and development needs. Beginning in week 9 the fetus synthesizes glycogen for
storage in the liver. Between 26 and 30 weeks the fetus begins to lay down
stores of brown fat in preparation for extrauterine cold
stress. Thermoregulation in the neonate requires increased metabolism and
adequate oxygenation.
The
gastrointestinal system is mature by 36 weeks. Digestive enzymes (except
pancreatic amylase and hpase) are present in
sufficient quantity to facilitate digestion; however, the neonate cannot digest
starches or fats efficiently. Little saliva is produced.
Renal system
The
kidneys form during the fifth week and begin to function approximately 4 weeks
later. Urine is excreted into the amniotic fluid and forms a major part of the
amniotic fluid volume. Oligohydramnios is indicative
of renal dysfunction. Because the placenta acts as the organ of excretion and
maintains fetal water and electrolyte balance, the fetus does not need
functioning kidneys while in utero. At birth,
however, the kidneys are required immediately for excretory and acid-base
regulatory functions.
A
fetal renal malformation can be diagnosed in utero. Corrective
or palliative fetal surgery may treat the malformation successfully, or plans
can be made for treatment immediately after birth (Jona,
1998).
At
term the fetus has fully developed kidneys. However, the glomerular
filtration rate is low, and the kidneys lack the ability to concentrate urine.
This makes the newborn more susceptible to both overhydration
and dehydration.
Most
newborns void within 24 hours of birth. With the loss of the swallowed amniotic
fluid and the metabolism of nutrients provided by the placenta, voidings for the first days of life are scanty until fluid
intake increases.
Neurologic system
The
nervous system originates from the ectoderm during the third week after
fertilization. The open neural tube forms during the fourth week. It initially
closes at what will be the junction of the brain and spinal cord, leaving both ends
open. The embryo folds in on itself lengthwise at this time, forming a head
fold in the neural tube at this junction. The cranial end of the neural tube
closes, then the caudal end closes. During week 5,
different growth rates cause more flexures in the neural tube, delineating
three brain areas: the forebrain, midbrain, and hindbrain.
The
forebrain develops into the eyes (cranial nerve II) and cerebral hemispheres.
The development of all areas of the cerebral cortex continues throughout fetal
life and into childhood. The olfactory system (cranial nerve I) and thalamus also
develop from the forebrain. Cranial nerves III and IV (oculomotor
and trochlear) form from the midbrain. The hindbrain
forms the medulla, the pons, the cerebellum, and the
remainder of the cranial nerves. Brain waves can be recorded on an
electroencephalogram by week 8.
The
spinal cord develops from the long end of the neural tube. Another ectodermal structure, the neural crest, develops into the
peripheral nervous system. By the eighth week, nerve fibers traverse throughout
the body. By week 11 or 12, the fetus makes respiratory movements, moves all
extremities, and changes position in utero. The fetus
can suck his or her thumb and swim in the amniotic fluid pool,
turn somersaults, and sometimes ties a knot in the umbilical cord. Sometime
between 16 and 20 weeks, when the movements are strong enough to be perceived by
the mother as "the baby moving," quickening has occurred. The
perception of movement occurs earlier in the multipara
than in the primipara. The mother also becomes aware
of the sleeping and waking cycles of the fetus.
Sensory
awareness. Purposeful movements of the fetus have
been demonstrated in response to a firm touch transmitted through the mother's
abdomen. Because the fetus can feel, invasive procedures to be done on a fetus
require anesthesia.
Fetuses
respond to sound by 24 weeks. Different types of music evoke different
movements. The fetus can be soothed by the sound of the mother's voice.
Acoustic stimulation can be used to evoke a fetal heart rate response. The fetus
becomes accustomed (i.e., habituates) to noises heard repeatedly. Hearing is
fully developed at birth.
The
fetus is able to distinguish taste. By the fifth month, when the fetus is
swallowing amniotic fluid, a sweetener added to the fluid causes the fetus to
swallow faster. The fetus also reacts to temperature changes. A cold solution placed
into the amniotic fluid can cause fetal hiccups.
The
fetus can see. Eyes have both rods and cones in the retina by the seventh
month. A bright light shone on the mother's abdomen in late pregnancy causes
abrupt fetal movements. During sleep time, rapid eye movements have been
observed similar to those occurring m children and adults while dreaming (Cole,
1997).
At
term the fetal brain is approximately one fourth the size of an adult brain. Neurologic development continues. Stressors on the fetus
and neonate (e.g., chronic poor nutrition or hypoxia, drugs, environmental
toxins, trauma, disease) cause damage to the central nervous system long after
the vulnerable embryonic time for malformations in other organ systems. Neurologic insult can result in cerebral palsy,
neuromuscular impairment, mental retardation, and learning disabilities.
Endocrine system
The
thyroid gland develops along with structures in the head and neck during the
third and fourth weeks. The secretion of thyroxine
begins during the eighth week. Maternal thyroxine
does not readily cross the placenta; therefore the fetus that does not produce
thyroid hormones will be born with congenital hypothyroidism. If untreated,
hypothyroidism can result in severe mental retardation. Screening for hypothyroidism
is typically included in the testing when screening for PKU after birth.
The
adrenal cortex is formed during the sixth week and produces hormones by the
eighth or ninth week. As term approaches, the fetus produces more cortisol. This is believed to aid in initiation of labor by
decreasing the maternal progesterone and stimulating production of
prostaglandins.
The
pancreas forms from the foregut during the fifth through eighth weeks. The
islets of Langerhans develop during the twelfth week.
Insulin is produced by the twentieth week. In infants of mothers with
uncontrolled diabetes, maternal hyperglycemia produces fetal hyperglycemia, stimulating
hyperinsulinemia and islet cell hyperplasia. This
results in a macrosomatic (large-sized) fetus. The hyperinsulinemia also blocks lung maturation, placing the neonate
at risk for respiratory distress and hypoglycemia when the maternal glucose
source is lost at birth. Control of the maternal glucose level before and
during pregnancy minimizes problems for the fetus and infant.
Reproductive system
Sex
differentiation begins in the embryo during the seventh week. Distinguishing
characteristics appear around the ninth week and are fully differentiated by
the twelfth week. When a Y chromosome is present, testes are formed. By the end
of the embryonic period, testosterone is being secreted and causes formation of
the male genitalia. By week 28 the testes begin descending into the scrotum.
After birth, low levels of testosterone continue to be secreted until the
pubertal surge.
The
female, with two X chromosomes, forms ovaries and female external genitalia. By
the sixteenth week, oogenesis has been established.
At birth the ovaries contain the female's lifetime supply of ova. Most female
hormone production is delayed until puberty. However, the fetal endometrium responds to maternal hormones, and withdrawal bleeding
or vaginal discharge (pseudomenstruation) may occur
at birth when these hormones are lost. The high level of maternal estrogen also
stimulates mammary engorgement and secretion of fluid ("witch's
milk") in newborn infants of both sexes.
Musculoskeletal system
Bones
and muscles develop from the mesoderm by the fourth week of embryonic
development. At that time the cardiac muscle is already beating. The mesoderm
next to the neural tube forms the vertebral column and ribs. The parts of the
vertebral column grow toward each other to enclose the developing spinal cord.
Ossification, or bone formation, begins. If there is a defect in the bony
fusion, spina bifida may occur. A large defect
affecting several vertebrae may allow the membranes and spinal cord to pouch out
from the back, producing neurologic deficits and skeletal
deformity.
The
flat bones of the skull develop during the embryonic period, and ossification continues
throughout childhood. At birth, connective tissue sutures exist where the bones
of the skull meet. The areas where more than two bones meet (called fontanels)
are especially prominent. The sutures and fontanels allow the bones of the
skull to mold, or move during birth, enabling the head to pass through the
birth canal.
The
bones of the shoulders, arms, hips, and legs appear in the sixth week as a
continuous skeleton with no joints. Differentiation occurs, producing separate
bones and joints. Ossification will continue through childhood to allow growth.
Beginning during the seventh week, muscles contract spontaneously. Arm and leg
movements are visible on ultrasound, although the mother does not perceive them
until sometime between 16 and 20 weeks.
Integumentary system
The
epidermis begins as a single layer of cells derived from the ectoderm at 4
weeks. By the seventh week, there are two layers of cells. The cells of the
superficial layer are sloughed and become mixed with the sebaceous gland secretions
to form the white, cheesy vernix caseosa,
the material that protects the skin of the fetus. The vernix
is thick at 24 weeks but becomes scant by term.
The
basal layer of the epidermis is the germinal layer, which replaces lost cells.
Until 17 weeks the skin is thin and wrinkled, with blood vessels visible
underneath. The skin thickens, and all layers are present at term. After 32
weeks, as subcutaneous fat is deposited under the dermis, the skin becomes less
wrinkled and red in appearance.
By
16 weeks the epidermal ridges are present on the palms of the hands, the
fingers, the bottom of the feet, and the toes. These handprints and footprints
are unique to that infant.
Hairs
form from the hair bulbs in the epidermis that project into the dermis. Cells
in the hair bulb keratinize to form the hair shaft. As the cells at the base of
the hair shaft proliferate, the hair grows to the surface of the epithelium. Very
fine hairs, called lanugo, appear first at 12 weeks
on the eyebrows and upper lip. By 20 weeks they cover the entire body. At this
time the eyelashes, eyebrows, and scalp hair are beginning to grow. By 28 weeks
the scalp hair is longer than the lanugo, which thins
and may disappear by term gestation.
Fingernails
and toenails develop from thickened epidermis at the tips of the digits
beginning during the tenth week. They grow slowly. Fingernails usually reach
the fingertips by 32 weeks, and toenails reach toetips
by 36 weeks.
Immunologic system
During
the third trimester, albumin and globulin are present in the fetus. The only
immunoglobulin (Ig) that crosses the placenta, IgG, provides passive acquired immunity to specific
bacterial toxins. The fetus produces IgM immunoglobulins by the end of the first trimester. These are
produced in response to blood group antigens, gramnegative
enteric organisms, and some viruses. IgA immunoglobulins are not produced by the fetus; however, colostrum, the precursor to breast milk, contains large amounts
of IgA and can provide passive immunity to the neonate
who is breastfed.
The
normal term neonate can fight infection, but not as effectively as an older
child. The preterm infant is at much greater risk for infection.
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