Conception, and Fetal Development

Conception, and Fetal Development

Prepared by assistant professor N.Petrenko, MD, PhD

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 Union of the sperm and ovum resulting in fertilization; formation of the one-celled zygote

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 Union of an ovum and sperm

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).

 

BOX 1 Potential Impact of Genetic Disease on Family and Community 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.). New York: Springer.

 

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 (Anderson, 1999).

 

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 (Box 2).

 

BOX 2 Protection of Patient Privacy 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 United States (Rice, 1998). It was expected that by the year 2005, the entire human genome would be mapped and that all of the 5 billion nucleic acid-base pairs and 100,000 genes would be identified. However, in June 2000, initial sequencing of the human genome was completed, well ahead of schedule. The map will facilitate study of hereditary diseases and will provide the potential for making changes at the gene level to treat or prevent hereditary diseases. Mapping is significant because there are more than 7000 known single-gene disorders (Munro, 1999).

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), Huntington disease, and polycystic kidney disease.

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.

 

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 (Box 3). A teratogen has the greatest effect on the organs and parts of an embryo during its periods of rapid differentiation. This occurs during the embryonic period, specifically from days 15 to 60. During the first 2 weeks of development, teratogens either have no effect on the embryo or have effects so severe that they cause miscarriage. Brain growth and development continue during the fetal period, and teratogens can severely affect mental development throughout gestation (Fig. 3).

 

Box 3 ETIOLOGY OF HUMAN MALFORMATIONS 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. St. Louis: Mosby.

 

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.]. Philadelphia: WB Saunders.)

 

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. St. Louis: Mosby. B, Adapted from Langley, L. et al. [1980]. Dynamic human anatomy and physiology [5th ed.]. New York: McGraw-Hill.)

 

 

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, Costa Mesa, CA.)

 

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.[. Philadelphia: WB Saunders.)

 

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.]. Philadelphia: WB Saunders.)

 

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.

.

Table 1 summarizes embryonic and fetal development
4 WEEKS	8 WEEKS	12 WEEKS	16 WEEKS	20 WEEKS	24 WEEKS	28 WEEKS	3O-31 WEEKS	36 AND 4O WEEKS
EXTERNAL APPEARANCE
Body flexed, C shaped; arm and leg buds present; head at right angles to body	Body fairly well formed; nose flat, eyes far apart; digits well formed; head elevating; tail almost disappeared; eyes, ears, nose, and mouth recognizable	Nails appearing; resembles a human;head erect but disproportionately large; skin pink, delicate	Head still dominant; face looks human; eyes, ears, and nose approach typical appearance on gross examination; arm/leg ratio proportionate; scalp hair appears	Vernix caseosa appears; lanugo appears; legs lengthen considerably; sebaceous glands appear	Body lean but fairly well proportioned; skin red and wrinkled; vernix caseosa present; sweat glands forming	Lean body, less wrinkled and red; nails appear	Subcutaneous fat beginning to collect; more rounded appearance; skin pink and smooth; has assumed birth position	Skin pink, body rounded; general lanugo disappearing; body usually plump 40 Weeks Skin smooth and pink; scant vernix caseosa; moderate to profuse hair; lanugo on shoulders and upper body only; nasal and alar cartilage apparent
CROWN-TO-RUMP MEASUREMENT; WEIGHT
0.4 to 0.5 cm; 0.4 g	2.5 to 3 cm; 2 g	6 to 9 cm; 19 g	11.5 to 13.5 cm; 100 g	16 to 18.5 cm; 300 g	23 cm; 600 g	27 cm; 1100 g	31 cm; 1800 to 2100 g	35 cm; 2200 to 2900 g
GASTROINTESTINAL SYSTEM
Stomach at midline and fusiform; conspicuous liver; esophagus short; intestine a short tube	Intestinal villi developing; small intestines coil within umbilical cord; palatal folds present; liver very large	Bile secreted; palatal fushion complete; intestines have withdrawn from cord and assume characteristic positions	Meconium in bowel; some enzyme secretion; anus open	Enamel and dentine depositing; ascending colon recognizable		Astragalus (talus, ankle bone) ossifies; weak, fleeting movements, minimum tone	Middle fourth phalanxes ossify; permanent teeth primordia seen; can turn head to side	Distal femoral ossification centers present; sustained, definite movements; fair tone; can turn and elevate head 40 Weeks Active, sustained movement; good tone; may lift head
MUSCULOSKELETAL SYSTEM
All somites present	First indication of ossification—occiput, mandible, and humerus; fetus capable of some movement; definitive muscles of trunk, limbs, and head well represented	Some bones well outlined, ossification spreading; upper cervical to lower sacral arches and bodies ossify; smooth muscle layers indicated in hollow viscera	Most bones distinctly indicated throughout body; joint cavities appear; muscular movements can be detected	Sternum ossifies; fetal movements strong enough for mother to feel		Lecithin forming on alveolar surfaces	US ratio = 1.2:	36 Weeks L/S ratio > 2:1 40 Weeks Pulmonary branching only two-thirds complete
CIRCULATORY SYSTEM
Heart develops, double chambers visible, begins to beat; aortic arch and major veins completed	Main blood vessels assume final plan; enucleated red cells predominate in blood	Blood forming in marrow	Heart muscle well developed; blood formation active in spleen		Blood formation increases in bone marrow and decreases in liver
RESPIRATORY SYSTEM
Primary lung buds appear	Pleural and pericardial cavities forming; branching bronchioles; nostrils closed by epithelial plugs	Lungs acquire definite shape; vocal cords appear	Elastic fibers appears in lungs; terminal and respiratory bronchioles appear	Nostrils reopen; primitive respiratory-like movements begin	Alveolar ducts and sacs present; lecithin begins to appear in amniotic fluid (weeks 26 to 27)
RENAL SYSTEM
Rudimentary ureteral buds appear	Earliest secretory tubules differentiating; bladder-urethra separates from rectum	Kidney able to secrete urine; bladder expands as a sac	Kidney in position; attains typical shape and plan					36 Weeks Formation of new nephrons ceases
NERVOUS SYSTEM
Well-marked midbrain flexure; no hindbrain or cervical flexures; neural groove closed	Cerebral cortex begins to acquire typical cells; differentiation of cerebral cortex, meninges, ventricular foramina, cerebrospinal fluid circulation; spinal cord extends entire length of spine	Brain structural configuration almost complete; cord shows cervical and lumbar enlargements; fourth ventricle foramina are developed; sucking present	Cerebral lobes delineated; cerebellum assumes some prominance	Brain grossly formed; cord myelination begins; spinal cord ends at level of first sacral vertebra (S1)	Cerebral cortex layered typically; neuronal proliferation in cerebral cortex ends	Appearance of cerebral fissures, convolutions rapidly appearing; indefinite sleep-wake cycle; cry weak or absent; weak suck reflex		36 Weeks End of spinal cord at level of third lumbar vertebra (L3); definite sleepwake cycle 40 Weeks Myelination of brain begins; patterned sleep-wake cycle with alert periods; cries when hungry or uncomfortable; strong suck reflex
SENSORY ORGANS
Eye and ear appearing as optic vessel and otocyst	Primordial choroid plexuses develops; ventricles large relative to cortex; development progressing; eyes converging rapidly; internal ear developing	Earliest taste buds indicated; characteristic organization of eye attained	General sense organs differentiated	Nose and ears ossify	Can hear	Eyelids reopen; retinal layers completed, light receptive; pupils capable of reacting to light	Sense of taste present; aware of sounds outside mother's body
GENITAL SYSTEM
Genital ridge appears (fifth week)	Testes and ovaries distinguishable; external genitalia sexless but begin to differentiate	Sex recognizable; internal and external sex organs specific	Testes in position for descent into scrotum: vagina open		Testes at inguinal ring in descent to scrotum		Testes descending to scrotum	40 Weeks Testes in scrotum; labia majora well developed

 

MULTIFETAL PREGNANCY

Twins

When two mature ova are produced in one ovarian cycle, both have the potential to be fertilized by separate sperm. This results in two zygotes, or dizygotic twins (Fig. 12). There are always two amnions, two chorions, and two placentas that may be fused together. These dizygotic, or fraternal, twins may be the same sex or different sexes and are genetically no more alike than siblings born at different times. Dizygotic twinning occurs in families, more often among African-American women than Caucasian women, and least often among Asian-American women. Dizygotic twinning increases in frequency with maternal age up to 35 years, with parity, and with the use of fertility drugs.

 

 

Fig. 12 Formation of dizygotic twins. There is fertilization of two ova, two implantations, two placentas, two chorions, and two amnions.

 

Identical twins, or monozygotic twins, develop from one fertilized ovum, which then divides (Fig. 13). They are the same sex and have the same genotype. If division occurs soon after fertilization, two embryos, two amnions, two chorions, and two placentas that may be fused will develop. Most often, division occurs between 4 and 8 days after fertilization, and there are two embryos, two amnions, one chorion, and one placenta. Rarely, division occurs after the eighth day following fertilization. In this case there are two embryos within a common amnion and a common chorion with one placenta. This often causes circulatory problems because the umbilical cords may tangle together, and one or both fetuses may die. If division occurs very late, cleavage may not be complete, and conjoined or "Siamese" twins could result. Monozygotic twinning occurs in approximately 1 of 250 births (Cunningham et al., 2001). There is no association with race, heredity, maternal age, or parity. Fertility drugs increase the incidence of monozygotic twinning.

 

 

Fig. 13 Formation of monozygotic twins. A, One fertilization: blastomeres separate, resulting in two implantations, two placentas, and two sets of membranes. B, One blastomere with two inner cell masses, one fused placenta, one chorion, and separate amnions. C, One blastomere with incomplete separation of cell mass resulting in conjoined twins.

 

Other multifetal pregnancies

The occurrence of multifetal pregnancies with three or more fetuses has increased with the use of fertility drugs and in vitro fertilization. Triplets occur in approximately 1 of 7600 pregnancies. They can occur from the division of one zygote into two, with one of the two dividing again, producing identical triplets. Triplets can also be produced from two zygotes, one dividing into a set of identical twins and the second zygote developing as a single fraternal sibling, or from three zygotes. Quadruplets, quintuplets, sextuplets, and so on have similar possible derivations.

 

 

NUTRIENT NEEDS BEFORE CONCEPTION

A healthful diet before conception is the best way to ensure that adequate nutrients are available for the developing fetus. Folic acid (folate) intake is of particular concern before conception and during early gestation, because neural tube defects (i.e., failure of the neural tube to close) are more common in infants of women with poor folic acid intake. It is estimated that the incidence of neural tube defects could be halved if all women had an adequate folic acid intake during this period (Butterworth & Bendich, 1996). All women capable of becoming pregnant are advised to consume 400 p,g of folic acid daily in fortified foods (e.g., ready-to-eat cereals and enriched grain products) or supplements, in addition to a diet rich in folic acid-containing foods: green leafy vegetables, whole grains, and meats.

Both maternal and fetal risks in pregnancy are increased when the mother is significantly underweight or overweight when pregnancy begins. Ideally, all women would achieve their desirable body weights before conception.

 

NUTRIENT NEEDS DURING PREGNANCY

Nutrient needs are determined, at least in part, by the stage of gestation in that the amount of fetal growth varies during the different stages of pregnancy. During the first trimester the synthesis of fetal tissues places relatively few demands on maternal nutrition. Therefore, during the first trimester, when the embryo/fetus is very small, the needs are only slightly increased over those before pregnancy. In contrast, the last trimester- is a period of noticeable fetal growth when most of the deposition of fetal stores of energy sources and minerals occurs. Basal metabolic rates, when expressed as kilocalories (kcal) per minute, are approximately 20% higher in pregnant women than in nonpregnant women. This increase includes the energy cost for tissue synthesis.

Dietary reference intakes (DRIs) are a new approach that the Food and Nutrition Board of the National Academy of Sciences has adopted to provide new nutritional recommendations for the people of the United States; Health Canada is also involved in this effort (Yates, Schlicker, & Suitor, 1998). The DRIs consist of recommended dietary allowances (RDAs) and adequate intakes (AIs), as well as guidelines for avoiding excessive nutrient intakes. RDAs are recommendations for daily nutritional intakes that meet the needs of almost all of the healthy members of the population. AIs are similar to the RDAs except that they are used when there are not enough data available to be certain that they meet the needs of the healthy population. The RDAs and the AIs include a wide variety of nutrients and food components, and they are divided into age, sex, and life-stage categories (e.g., infancy, pregnancy, lactation). They can be used as goals in planning the diets of individuals (Table 1).

 

TABLE 10-1 Nutritional RBcommendations During Pregnancy and Lactation

NUTRIENT

(UNIT)           RECOMMENDATION

FOR NONPREGNANT FEMALE*            RECOMMENDATION

FOR PREGNANCY*           RECOMMENDATION

FOR LACTATION* ROLE IN RELATION

TO PREGNANCY AND LACTATION     FOOD/FOOD SOURCES

Energy            Variable          First trimester, same as nonpregnant; second and third trimesters, nonpregnant + 300      Nonpregnant+ 500     Growth of fetal and maternal issues; milk production         Carbohydrate, fat, protein

Protein            50        60        65        Synthesis of the products of conception; growth of maternal tissue and expansion of blood volume; secretion of milk protein during lactation       Meats, eggs, cheese, yogurt, legumes (dry beans and peas, peanuts), nuts, grains

MINERALS

Calcium (mg)  1300/1000       1300/1000       1300/1000       Fetal and infant skeleton and tooth formation; maintenance of maternal bone and tooth mineralization      Milk, cheese, yogurt, sardines or other fish eaten with bones left in, deep green leafy vegetables except spinach or Swiss chard, tofu, baked beans

Phosphorus (mg)        1250/700         1250/700         1250/700         Fetal and infant skeleton and ooth formation        Milk, cheese, yogurt, meats, whole grains, nuts, legumes

Iron (mg)        15        30        15        Maternal hemoglobin formation, fetal liver iron storage      Liver, meats, whole or enriched breads and cereals, deep green leafy vegetables, legumes, dried fruits

Zinc (mg)

            12        15        19        Component of numerous enzyme systems; possibly important in preventing congenital malformations       Liver, shellfish, meats, whole grains, milk

Iodine (|xg)

            150      175      200      Increased maternal metabolic

rate      Iodized salt, seafood, milk and

milk products, commercial

yeast breads, rolls, donuts

Magnesium (mg)        360/320

            400/360           360/320

            Involved in energy and

protein metabolism, tissue

growth, muscle action            Nuts, legumes, cocoa, meats,

whole grains

FAT-SOLUBLE VITAMINS

A (RE)            800      800      1300    Essential for cell development,

tooth bud formation,

bone growth

            Deep green leafy vegetables,

dark yellow vegetables and

fruits, chili peppers, liver,

fortified margarine and

butter

 

D (gμ) 5          5          5          Involved in absorption of

calcium and phosphorus,

improves mineralization         Fortified milk and margarine,

egg yolk, butter, liver,

seafood

E(mg)

            8          10        12        Antioxidant (protects cell

membranes from damage),

especially important for

preventing breakdown of

RBCs

            Vegetable oils, green leafy

vegetables, whole grains,

liver, nuts and seeds,

cheese, fish

WATER-SOLUBLE VITAMINS

C (mg) 60        70        95        Tissue formation and

integrity, formation of connective

tissue, enhancement

of iron absorption

            Citrus fruits, strawberries,

melons, broccoli, tomatoes,

peppers, raw deep green

leafy vegetables

Folic acid (gμ)

            400      600      500      Prevention of neural tube

defects, support increased

maternal RBC formation

            Fortified ready-to-eat cereals

and other grains, green leafy

vegetables, oranges, broccoli,

asparagus, artichokes,

liver

Thiamine (mg)            1.0/1.1 1.4       1.5       Involved in energy metabolism         Pork, beef, liver, whole or

enriched grains, legumes

Riboflavin (mg)          1.0/1.1 1.4       1.6       Involved in energy and

protein metabolism     Meat, liver, deep green vegetables,

whole grains

Niacin (mg)     14        18        17        Involved in energy metabolism         Meat, fish, poultry, liver, whole

or enriched grains, peanuts

Pyridoxine (B6) (mg) 1.2/1.3 1.9       2.0       Involved in protein metabolism         Meat, liver, deep green vegetables,

whole grains

B12 (|xg)         2.4       2.6       2.8       Production of nucleic acids

and proteins, especially

important in formation of

RBC and neural functioning  Milk and milk products, egg,

meat, liver, fortified soy milk

 

Recommendations are the new dietary reference intakes (RDAor Al, see text) where available (Food and Nutrition Board, National Academy of Sciences, Institute of Medicine. [1998]. Recommended levels for individual intake, B vitamins, and choline. Washington, DC: National Academy Press; Standing Committee on the Scientific Evaluation of Dietary Reference Intakes, Food and Nutrition Board, Institute of Medicine. [1997]. Dietary reference intakes: Calcium, phosphorus, magnesium, vitamin D, and fluoride. Washington, DC: National Academy Press). Where DRI are not yet available, the values are taken from Food and Nutrition Board (1989).

RBC, Red blood cells.

*When two values appear, separated by a diagonal slash, the first is for females <19 years and the second is for those 19 to 50 years old.

ENERGY NEEDS

Energy (kilocalories; abbreviated kcal) needs are met by carbohydrate, fat, and protein in the diet. No specific recommendations exist for the amount of carbohydrate and fat in the diet of the pregnant woman. However, intake of these nutrients should be adequate to support the recommended weight gain. Although protein can be used to supply energy, its primary role is to provide amino acids for the synthesis of new tissues (see discussion on protein later in this chapter). The RDA during the second and third trimesters of pregnancy is 300 kcal greater than prepregnancy needs; very underweight or active women may require more than 300 additional kcal to sustain the desired rate of weight gain.

 

Weight gain

The optimal weight gain during pregnancy is not known precisely. It is known, however, that the amount of weight gained by the mother during pregnancy has an important bearing on the course and outcome of pregnancy.

Adequate weight gain reduces the risk of delivering a small for gestational age (SGA) or preterm infant.

The desirable weight gain during pregnancy varies among individual women. Maternal and fetal risks in pregnancy are increased when the mother is either significantly underweight or overweight before pregnancy and when weight gain during pregnancy is either too low or too high. Women with inadequate weight gain have an increased risk of delivering an infant with intrauterine growth restriction (IUGR). Greater-than-expected weight gain during pregnancy may occur for many reasons, including multiple gestation, edema, pregnancy-induced hypertension, and overeating. When obesity is present (either preexisting or developed during pregnancy), there is an increased likelihood of macrosomia and fetopelvic disproportion, operative birth, birth trauma, and infant death. Obese women are more likely to have hypertension and diabetes, and their risk of giving birth to a child with a major congenital defect is double that of normal-weight women (Prentice & Goldberg, 1996). The cost of pregnancy in an obese woman has been estimated to be triple that of a normal-weight woman (Prentice & Goldberg, 1996).

The primary factor to consider in making a weight gain recommendation is the appropriateness of the prepregnancy weight for the woman's height. A commonly used method of evaluating the appropriateness of weight for height is the body mass index (BMI), which is calculated by the following formula:

BMI = Weight/Height2

where the weight is in kilograms and height is in meters. Thus for a woman who weighed 51 kg before pregnancy and is 1.57 m tall:

BMI = 51/(1.57)2, or 20.7

BMI can be classified into the following categories: less than 19.8, underweight or low; 19.8 to 26.0, normal; 26.0 to 29.0, overweight or high; and greater than 29.0, obese.

For women with single fetuses, current recommendations are that women with a normal BMI should gain 11.5 to 16 kg during pregnancy, underweight women should gain 12.5 to 18 kg, overweight women should gain 7 to 11.5 kg, and obese women should gain at least 7 kg. Adolescents are encouraged to strive for weight gains at the upper end of the recommended range for their BMI because it appears that the fetus and the still-growing mother compete for nutrients. The risk of mechanical complications at birth is reduced if the weight gain of short adult women (i.e., less than 157 cm) is near the lower end of their recommended range. In twin gestations, gains of approximately 16 to 20 kg appear to be associated with the best outcomes (Ellings, Newman, & Bower, 1998).

 

PATTERN OF WEIGHT GAIN

Weight gain should take place throughout pregnancy. The risk of delivering an SGA infant is greater when the weight gain early in pregnancy has been poor. The likelihood of preterm birth is greater when the gains during the last half of pregnancy have been inadequate. These risks exist even when the total gain for the pregnancy is in the recommended range.

The optimal rate of weight gain depends on the stage of pregnancy. During the first and second trimesters, growth takes place primarily in maternal tissue; during the third trimester, growth occurs primarily in fetal tissues. During the first trimester there is an average total weight gain of only 1 to 2.5 kg. Thereafter the recommended weight gain increases to approximately 0.4 kg per week for a woman of normal weight (Fig. 2). The recommended weekly weight gain for overweight women during the second and third trimesters is 0.3 kg, and for underweight women it is 0.5 kg. The recommended caloric intake corresponds to this pattern of gain. For the first trimester there is no increment; during the second and third trimesters an additional 300 kcal/day over the prepregnant intake is recommended. The amount of food providing 300 kcal is not great. It can be provided by one additional serving from each of the following groups: milk, yogurt, or cheese (all skim milk products); fruits; vegetables; and bread, cereal, rice, or pasta.

 

 

Fig. 2 Prenatal weight gain chart for plotting weight gain of normal-weight women. Young adolescents, African-American women, and smokers should aim for the upper end of the recommended range; short women (less than 157 cm) should strive for gains at the lower end of the range.

 

The reasons for an inadequate weight gain (less than 1 kg per month for normal-weight women or less than 0.5 kg per month for obese women during the last two trimesters) or excessive weight gain (more than 3 kg per month) should be evaluated thoroughly. Possible reasons for deviations from the expected rate of weight gain include measurement or recording errors, differences in weight of clothing or time of day, and accumulation of fluids, as well as inadequate or excessive dietary intake. An exceptionally high gain is likely to be caused by an accumulation of fluids, and a gain of more than 3 kg in a month, especially after the twentieth week of gestation, often heralds the development of pregnancy-induced hypertension.

 

HAZARDS OF RESTRICTING ADEQUATE WEIGHT GAIN

An obsession with thinness and dieting permeates the North American culture. Slender, figure-conscious women may find it difficult to make the transition from guarding against weight gain before pregnancy to valuing weight gain during pregnancy. In counseling these women, the nurse can emphasize the positive effects of good nutrition, as well as the adverse effects of maternal malnutrition (manifested by poor weight gain) on infant growth and development. This counseling includes information on the components of weight gain during pregnancy (Fig. 3) and the amount of this weight that will be lost at birth. Early in a woman's pregnancy, explaining ways to lose weight in the postpartum period helps relieve her concerns. Because lactation can help to reduce maternal energy stores gradually, this provides an opportunity to promote breastfeeding.

 

 

Fig. 3 Components of maternal weight gain at 40 weeks of gestation. (Modified from Worthington-Roberts, B., & Williams, S. [1997]. Nutrition in pregnancy and lactation [6th ed.]. Dubuque, IA: Brown & Benchmark.)

 

Pregnancy is not a time to diet. Even overweight or obese pregnant women need to gain at least enough weight to equal the weight of the products of conception (i.e., fetus, placenta, and amniotic fluid). If they limit their caloric intake to prevent weight gain, they may also excessively limit their intake of important nutrients. Moreover, dietary restriction results in catabolism of fat stores, which in turn augments the production of ketones. The longterm effects of mild ketonemia during pregnancy are not known, but ketonuria has been found to be correlated with the occurrence of preterm labor. It should be stressed to obese women, and to all pregnant women, that the quality of the weight gain is important, with emphasis on the consumption of nutrient-dense foods and the avoidance of empty-calorie foods.

Weight gain is important, but pregnancy is not an excuse for uncontrolled dietary indulgence. Excessive weight gained during pregnancy may be difficult to lose after pregnancy, thus contributing to chronic overweight or obesity, an etiologic factor in a host of chronic diseases, including hypertension, diabetes mellitus, and arteriosclerotic heart disease. The woman who gains 18 kg or more during pregnancy is especially at risk.

 

Protein

Protein, with its essential constituent nitrogen, is the nutritional element basic to growth. Adequate protein is essential to meet increasing demands in pregnancy. These demands arise from the rapid growth of the fetus; the enlargement of the uterus and its supporting structures, mammary glands, and placenta; an increase in maternal circulating blood volume and the subsequent demand for increased amounts of plasma protein to maintain colloidal osmotic pressure; and the formation of amniotic fluid.

Milk, meat, eggs, and cheese are complete protein foods with a high biologic value. Legumes (dried beans and peas), whole grains, and nuts are also valuable sources of protein. In addition, these protein-rich foods are a source of other nutrients such as calcium, iron, and B vitamins; plant sources of protein often provide needed dietary fiber. The recommended daily food plan (Table 2) is a guide to the amounts of these foods that would supply the quantities of protein needed. The recommendations provide for only a modest increase in protein intake over the prepregnant levels in adult women. Protein intake in many people in the United States is relatively high, so many women may not need to increase their protein intake at all during pregnancy. Three servings of milk, yogurt, or cheese (four for adolescents) and 5 to 6 ounces (140 to 168 g) (two servings) of meat, poultry, or fish supply the recommended protein for the pregnant woman. Additional protein is provided by vegetables and breads, cereals, rice, and pasta. Pregnant adolescents, women from impoverished backgrounds, and women adhering to unusual diets such as a macrobiotic (highly restricted vegetarian) diet are those whose protein intake is most likely to be inadequate. The use of high-protein supplements is not recommended because they have been associated with an increased incidence of preterm births.

 

Table 2 Daily Food Guide for Pregnancy and Lactation

 

 

Fluids

Water is the main substance of cells, blood, lymph, amniotic fluid, and other vital body fluids and is essential during the exchange of nutrients and waste products across cell membranes. It also aids in maintaining body temperature. A good fluid intake promotes good bowel function, which is sometimes a problem during pregnancy. Dehydration may increase the risk of cramping/contractions and preterm labor. The recommended daily intake is 6 to 8 glasses (1500 to 2000 ml) of fluid. Water, milk, and juices are good sources of fluids.

Women who consume more than 300 mg of caffeine daily (equivalent to 500 to 750 ml of coffee) are at increased risk of miscarriage and of delivering infants with IUGR. Caffeine's ill effects have been proposed to result from vasoconstriction of the blood vessels supplying the uterus or interference with cell division in the developing fetus (Hinds et al., 1996). Consequently, caffeine-containing products, including caffeinated coffee, tea, soft drinks, and cocoa beverages, should be avoided or consumed only in limited quantities.

Aspartame (e.g., Nutrasweet, Equal) and acesulfame K (e.g., Sweet One), artificial sweeteners commonly used in low- or no-calorie beverages, have not been found to have adverse effects on the normal mother or fetus, but aspartame use should be avoided by pregnant women who are homozygous for phenylketonuria (PKU).

 

Minerals and Vitamins

In general, the nutrient needs of pregnant women, except perhaps the need for iron, can be met through dietary sources. Counseling about the need for a varied diet rich in vitamins and minerals should be a part of every pregnant woman's early prenatal care and should be reinforced throughout pregnancy. However, supplements of certain nutrients (listed in the following discussion) are recommended whenever the woman's diet is very poor or whenever significant nutritional risk factors are present. Nutritional risk factors in pregnancy are listed in Box 1.

 

BOX 1 Indicators of Nutritional Risk in Pregnancy

Adolescence

Frequent pregnancies: three within 2 years

Poor fetal outcome in a previous pregnancy

Poverty

Poor diet habits with resistance to change

Use of tobacco, alcohol, or drugs

Weight at conception under or over normal weight

Problems with weight gain

Any weight loss

Weight gain of less than 1 kg/mo after the first trimester

Weight gain of more than 1 kg/wk after the first trimester

Multifetal pregnancy

Low hemoglobin or hematocrit values (or both)

 

Iron

Iron is needed both to allow for transfer of adequate iron to the fetus and to permit expansion of the maternal red blood cell (RBC) mass. Beginning in the latter part of the first trimester the blood volume of the mother increases steadily, peaking at approximately 1500 ml more than in the nonpregnant state. In twin gestations, the increase is at least 500 ml greater than in pregnancies with single fetuses. Plasma volume increases more than RBC mass. The relative excess of plasma causes a modest decrease in the hemoglobin concentration and hematocrit, known as physiologic anemia of pregnancy. This is a normal adaptation during pregnancy.

However, poor iron intake and absorption, which can result in iron deficiency anemia, is relatively common among women in the childbearing years. It affects nearly one fifth of the pregnant women in industrialized countries. The maternal mortality rate is increased among anemic women, who are poorly prepared to tolerate hemorrhage at the time of birth. In addition, anemic women may have a greater likelihood of cardiac failure during labor, postpartum infections, and poor wound healing. The fetus is also affected by maternal anemia. The risk of preterm birth is greater in anemic women, and fetal iron stores may also be reduced by maternal anemia (Allen, 2000). Anemia is more common among adolescents and African-American women than among adult Caucasian women.

Evidence supports the recommendation that all pregnant women receive a daily iron supplement (Allen, 2000). (Iron supplements may be poorly tolerated during the nausea that is prevalent in the first trimester.) If iron deficiency anemia (as manifested by low levels of hematocrit or hemoglobin and serum ferritin) is present, higher dosages are required. Certain foods taken with an iron supplement can promote or inhibit absorption of iron. Even when a woman is taking an iron supplement, she should include good food sources of iron in her daily diet (see Table 1).

 

Calcium

There is no increase in the DRI of calcium during pregnancy and lactation, in comparison with the recommendation for the nonpregnant woman (see Table 1). The DRI (1000 mg daily for women 19 and older and 1300 mg for those younger than 19) appears to provide sufficient calcium for fetal bone and tooth development to proceed while maintaining maternal bone mass.

Milk and yogurt are especially rich sources of calcium, providing approximately 300 mg per cup (240 ml). Nevertheless, many women do not consume these foods or do not consume adequate amounts to provide the recommended intakes of calcium. One problem that can interfere with milk consumption is lactose intolerance, the inability to digest milk sugar (lactose) caused by the absence of the lactase enzyme in the small intestine. Lactose intolerance is relatively common in adults, particularly African-Americans, Asians, Native Americans, and Eskimos. Milk consumption may cause abdominal cramping, bloating, and diarrhea in such people. Yogurt, sweet acidophilus milk, buttermilk, cheese, chocolate milk, and cocoa may be tolerated even when fresh fluid milk is not. Commercial products that contain the lactase enzyme (e.g., Lactaid) are available in pharmacies and many supermarkets. The lactase in these products hydrolyzes, or digests, the lactose in milk, making it possible for lactose-intolerant people to drink milk.

In some cultures, adults rarely drink milk. For example, Puerto Ricans and other Hispanic people may use it only as an additive in coffee. Pregnant women from these cultures may need to consume nondairy sources of calcium. Vegetarian diets may also be deficient in calcium (Box 2). If calcium intake appears low and the woman does not change her dietary habits despite counseling, a daily supplement containing 600 mg of elemental calcium may be needed. Calcium supplements may also be recommended when a pregnant woman experiences leg cramps caused by an imbalance in the calcium/phosphorus ratio.

 

BOX 2 Calcium Sources for Women Who Do Not Drink Milk

Each of the following provides approximately the same amount of calcium as 1 cup of milk:

FISH

3 oz can of sardines

4V2 oz can of salmon (if bones are eaten)

BEANS AND LEGUMES

3 cups of cooked dried beans

2VS cups of retried beans

2 cups of baked beans with molasses

1 cup of tofu (calcium is added in processing)

GREENS

1 cup of collards

1V2 cups of kale or turnip greens

BAKED PRODUCTS

3 pieces of cornbread

3 English muffins

4 slices of French toast

2 (7 inch diameter) waffles

FRUITS

11 dried figs

1V8 cups of orange juice with calcium added

SAUCES

3 oz of pesto sauce

5 oz of cheese sauce

 

Sodium

During pregnancy the need for sodium increases slightly, primarily because the body water is expanding (e.g., the expanding blood volume). Sodium is essential for maintaining body water balance. Grain, milk, and meat products, which are good sources of nutrients needed during pregnancy, are significant sources of sodium.

In the past, dietary sodium was routinely restricted in n effort to control the peripheral edema that commonly occurs during pregnancy. However, it is now recognized that moderate peripheral edema is normal in pregnancy, occurring as a response to the fluid-retaining effects of elevated levels of estrogen. An excessive emphasis on sodium restriction may make it difficult for pregnant women to achieve an adequate diet. In addition, restriction of sodium intake may stress the adrenal glands and the kidney as they attempt to retain adequate sodium. In general, sodium restriction is necessary only if the woman has a medical condition such as renal or liver failure or hypertension.

Excessive intake of sodium is discouraged during pregnancy just as it is in nonpregnant women, because it may contribute to abnormal fluid retention and edema. Table salt (sodium chloride) is the richest source of sodium. Most canned foods contain added salt unless the label pecifically states otherwise. Large amounts of sodium are also found in many processed foods, including meats (e.g., smoked or cured meats, cold cuts, corned beef), baked goods, mixes for casseroles or grain products, soups, and condiments. Products low in nutritive value and excessively high in sodium include pretzels, potato and other chips, pickles, ketchup, prepared mustard, steak and Worcestershire sauces, some soft drinks, and bouillon. A moderate sodium intake can usually be achieved by salting food lightly in cooking, adding no additional salt at the table, and avoiding low-nutrient/high-sodium foods.

 

Zinc

Zinc is a constituent of numerous enzymes involved in major metabolic pathways. Zinc deficiency is associated with malformations of the central nervous system in infants. When large amounts of iron and folic acid are consumed, the absorption of zinc is inhibited and serum zinc levels are reduced as a result. Because iron and folic acid supplements are commonly prescribed during pregnancy, pregnant women should be encouraged to consume good sources of zinc daily (see Table 10-1). Women with anemia who receive high-dose iron supplements also need supplements of zinc (King, 2000).

 

Fluoride

The effect of prenatal fluoride supplementation on tooth development in the infant is not fully known. However, it appears that prenatal fluoride supplementation has little effect on the incidence and prevalence of tooth decay (Leverett et al., 1997). No increase in fluoride intake over the nonpregnant DRI is currently recommended during pregnancy (Standing Committee, 1997).

 

Fat-soluble vitamins

Fat-soluble vitamins (i.e., vitamins A, D, E, and K) are stored in the body tissues. With chronic overdoses, these vitamins can reach toxic levels. Because of the high potential for toxicity, pregnant women are advised to take fatsoluble vitamin supplements only as prescribed. Vitamins A and D deserve special mention.

Adequate intake of vitamin A is needed so that sufficient amounts can be stored in the fetus. However, dietary sources can readily supply sufficient amounts. Congenital malformations have occurred in infants of mothers who took excessive amounts of vitamin A during pregnancy, and thus supplements are not recommended for pregnant women. Vitamin A analogs such as isotretinoin (Accutane), which are prescribed for the treatment of cystic acne, are a special concern. Isotretinoin use during early pregnancy has been associated with an increased incidence of heart malformations, facial abnormalities, cleft palate, hydrocephalus, and deafness and blindness in the infant, as well as an increased risk of miscarriage. Topical agents such as tretinoin (Retin-A) do not appear to enter the circulation in any substantial amounts, but their safety in pregnancy has not been confirmed.

Vitamin D plays an important role in absorption and metabolism of calcium. The main food sources of this vitamin are enriched or fortified foods such as milk and ready-to-eat cereals. Vitamin D is also produced in the skin by the action of ultraviolet light (in sunlight). Severe deficiency may cause neonatal hypocalcemia and tetany, as well as hypoplasia of the tooth enamel. Women with lactose intolerance and those who do not include milk in their diet for any reason are at risk for vitamin D deficiency. Other risk factors are dark skin, habitual use of clothing that covers most of the skin, and living in northern latitudes where sunlight exposure is limited, especially during the winter.

 

Water-soluble vitamins

Body stores of water-soluble vitamins are much smaller than those of fat-soluble vitamins; the water-soluble vitamins, in contrast to fat-soluble vitamins, are readily excreted in the urine. Therefore good sources of water-soluble vitamins must be consumed frequently, and toxicity with overdose is less likely than with fat-soluble vitamins.

Because of the increase in RBC production during pregnancy, as well as the nutritional requirements of the rapidly growing cells in the fetus and placenta, pregnant women should consume approximately 50% more folic acid than nonpregnant women, or approximately 600 /jug daily. This increased need for folic acid continues during lactation (Bailey & Gregory, 1999). In the United States, all enriched grain products (this includes most white breads, flour, and pasta) must contain folic acid at a level of 1.4 mg per kilogram of flour. This level of fortification supplies approximately 0.1 mg of folic acid daily in the average American diet (USDHHS, FDA, 1996). All women of childbearing potential need careful counseling about including good sources of folic acid in their diet (Tinkle & Sterling, 1997).

Pyridoxine, or vitamin B6, is involved in protein metabolism. Although levels of a pyridoxine-containing enzyme have been reported to be low in women with pregnancyinduced hypertension, there is no evidence that supplementation prevents or corrects the condition. No supplement is recommended routinely, but women with poor diets and those at nutritional risk (see Box 1) may need a supplement. Supplementation is related to a lowered incidence of dental decay in pregnant women (Mahomed & Gulmezoglu, 2000).

Vitamin C, or ascorbic acid, plays an important role in tissue formation and enhances the absorption of iron. The vitamin C needs of most women are readily met by a diet that includes at least five servings per day of fruits and vegetables (Levine et al., 1999) (see Table 1), but women who smoke need more. For women at nutritional risk, a supplement is recommended. However, if the mother takes excessive doses of this vitamin during pregnancy, a vitamin C deficiency may develop in the infant after birth.

 

Nutrient supplements

Food can and should be the normal vehicle to meet the additional needs imposed by pregnancy (excepting iron, for which a supplemental dose is recommended). However, some women chronically consume diets that are deficient in necessary nutrients and, for whatever reason, may be unable to change this intake. For these women a supplement should be considered. It is important that the pregnant woman understand that the use of a vitamin/mineral supplement does not lessen the need to consume a nutritious, well-balanced diet.

 

OTHER NUTRITIONAL ISSUES DURING

PREGNANCY

Pica and food cravings

Pica is the practice of consuming nonfood substances (e.g., clay, dirt, laundry starch) or excessive amounts of foodstuffs low in nutritional value (e.g., cornstarch, ice, baking powder, soda). Pica is often influenced by the woman's cultural background. In the United States it appears to be most common among African-American women, women from rural areas, and women with a family history of pica. Regular and heavy consumption of low-nutrient products may cause more nutritious foods to be displaced from the diet, and the items consumed may interfere with the absorption of nutrients, especially minerals. Women with pica have lower hemoglobin levels than those without pica (Rainville, 1998). The possibility of pica must be considered when pregnant women are found to be anemic, and the nurse should provide counseling about the health risks associated with pica. The existence of pica, as well as details of the type and amounts of products ingested, is likely to be discovered only by the sensitive interviewer who has developed a relationship of trust with the woman. It has been proposed that pica and food cravings (e.g., the urge to consume ice cream, pickles, pizza) during pregnancy are caused by an innate drive to consume nutrients missing from the diet. However, research has not supported this hypothesis.

 

Adolescent pregnancy

Many adolescent females have diets that fall below the recommended intakes of key nutrients, including energy, calcium, and iron. Teens have lower BMIs than adults and are at risk for having babies of lower birth weight than adult women (Buschman, Foster, & Vickers, 2001).

Pregnant adolescents and their infants are at increased risk of complications during pregnancy and parturition. Growth of the pelvis is delayed in comparison to growth in stature, and this helps explain why cephalopelvic disproportion and other mechanical problems associated with labor are common among young adolescents. Competition between the growing adolescent and the fetus for nutrients may also contribute to some of the poor outcomes apparent in teen pregnancies. Pregnant adolescents are encouraged to choose a weight gain goal at the upper end of the range for their BMI (see Research box).

 

RESEARCH

Weight Gain and Birth Weight in the Pregnant Young Teen

The weight of newborns is correlated with immediate survival and with long-term health. Pregnancy in adolescence is especially vulnerable to a low-birthweight outcome, possibly because the adolescent starts pregnancy with a lower body mass index (BMI), has poor nutrition, or is still growing herself. Low birth weight for gestation indicates pathology of the fetus, mother, or placenta. Birth weight of the baby is dependent on weight gain of the mother during pregnancy. Girls 16 years of age and older share the same weight gain patterns and pregnancy outcomes as adult women. Younger teens, however, do not fit this pattern.

Studying a region of Scotland with a high rate of teen pregnancy, researchers retrospectively compared 104 pregnant adolescents, ages 13 to 15 years, with a control group of 150 pregnant adults, ages 25 to 30 years. The researchers calculated the "prepregnancy BMI" based on weight and height at first visit up to 16 weeks of gestation. The "end-of-pregnancy BMI" calculation came from the original height and the recorded weight at 36 or more weeks of gestation. Analysis of the data confirmed that the adolescents started and ended pregnancy with lower BMIs than the adults. For both groups, a higher end-of-pregnancy BMI correlated to higher birth weights. Adolescents gained as much weight in proportion to body size as adult women, yet still delivered lower-birth-weight babies. It was noteworthy that 30% of the adolescents smoked, compared with 18% of adults, which also may have affected birth weight.

IMPLICATIONS FOR PRACTICE

Nurses who encounter young pregnant teens need to be aware of their special nutritional needs. Involving the nutritionist and the family members who prepare the meals may assist the teen in eating the right kinds of food for a healthy baby outcome. Referrals to special "Teen OB" clinics and peer support groups may provide conducive environments for teens to be educated about nutrition, smoking cessation, and the importance of preventing low-birth-weight babies.

Source: Buschman, N., Foster, G., & Vickers, P. (2001). Adolescent girls and their babies: Achieving optimal birthweight. Gestational weight gain and pregnancy outcome in terms of gestation at delivery and infant birth weight: A comparision between adolescents under 16 and adult women. Child Care Health Dev, 27(1) 163-171.