10.4 Fetal Growth and Development

Inheritance

Nursing care in the perinatal period requires the nurse to understand and explain basic patterns of inheritance to persons considering pregnancy or who are already pregnant. The patterns of inherited traits are the result of dominant or recessive genes. A dominant gene (brown eye color) will mask the trait of a recessive gene (blue eye color) in a person. Autosomal dominant disorders occur when the mutated gene produces the disorder when present in the heterozygous state. Autosomal recessive disorders occur when the mutated gene produces the disorder only when present in the homozygous state. It takes a pair of recessive genes (one from each parent) for the recessive trait to appear in the offspring.

Inherited conditions are also linked to the X and Y chromosomes. The conditions occur based on whether the gene is located only on the X or Y chromosome and if the gene is dominant or recessive. X-linked dominant disorders happen when the mutated gene is located only on the X chromosome and the disorder presents in the heterozygous state. X-linked recessive disorders happen when the mutated gene is located only on the X chromosome and the disorder presents only in the homozygous state. The mutated gene is located only on the Y chromosome in Y-linked disorders. The Y chromosome is paired with the X chromosome during fertilization to produce XY offspring. All XY offspring will have the disorder or trait. Figure 10.12 illustrates these different patterns of inheritance.

Figure 10.12 Patterns of Inheritance (a) When only one biological parent carries a dominant mutant gene, each child has a 50 percent chance of being affected and a 50 percent chance of not being affected. (b) When both biological parents carry a recessive mutant gene, each child has a 25 percent chance of being affected, a 50 percent chance of being a carrier, and a 25 percent chance of not being affected. (c) When the XY parent has the X-linked dominant disorder and the XX biological parent is not affected, 100 percent of the XX offspring will be carriers, and 100 percent of the XY offspring will be unaffected. (d) When the XY biological parent is not affected and the XX biological parent carries the recessive X-linked disorder or trait, the XX offspring will have a 50 percent chance of being a carrier and a 50 percent chance of being unaffected. The XY offspring will have a 50 percent chance of being affected and a 50 percent chance of not being affected. (modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0)

Chromosome Abnormalities

Some genetic disorders are the result of inherited abnormal chromosomes. However, most chromosomal disorders are not inherited. Instead, the abnormality occurs during the formation of the egg or the sperm, immediately after fertilization, or during embryonic development. Abnormalities in the chromosomes of the egg or sperm show up in every cell of the person’s body. When abnormalities in the chromosomes occur immediately after fertilization during mitosis, the abnormality shows as mosaicism, in which the abnormality in the chromosome is not present in every cell. In the United States, 1 out of 150 live-born newborns has chromosomal abnormalities (March of Dimes, 2022), and 50 percent of spontaneous abortions (miscarriages) have chromosome abnormalities (Cleveland Clinic, 2023). Health care providers use a karyotype, an image showing the results of a chromosome analysis, to identify anomalies.

A person may have more or fewer than the normal 46 chromosomes. Trisomy 21, Down syndrome, is the most commonly known trisomy abnormality, in which a third copy of the entire 21st chromosome is added. Trisomy 18 and Trisomy 13 also feature an additional entire chromosome. Turner syndrome occurs when an entire X chromosome is missing (Figure 10.13). In Klinefelter syndrome, an extra X chromosome, XXY, is present.

Figure 10.13 Karyotype of Turner Syndrome Aneuploidy is depicted in a karyotype of the chromosome analysis showing Turner syndrome. (credit: Wessex Reg. Genetics Centre/Wellcome Collection, CC BY 4.0)

Chromosome structure can be changed in multiple ways. Deletions occur when part of the chromosome is missing. Duplications occur when the entire chromosome or part of a chromosome is duplicated. Inversions are when a part of the chromosome breaks off and reattaches upside down. Rings are when a part of the chromosome breaks away and reattaches to form a ring. Translocations occur when a part of a chromosome is missing and the missing part has attached itself to a different chromosome altogether. These structural abnormalities can affect several genes with unpredictable effects on the affected person’s body structure and physiology.

Genetic Counseling

Genetic counseling is the provision of information about inherited conditions, disorders, or abnormalities affecting an individual or a family. The information is provided by a trained professional. Perinatally, counseling is provided to biological parents prior to conception, after a prenatal screening for or diagnosis of a chromosome or genetic disorder, and after the birth of a newborn with a chromosome or genetic or another congenital anomaly. The purpose of genetic counseling is to determine risk for genetic abnormalities, to answer questions about prognosis and long-term care of these conditions, to discuss legal and ethical issues, and to provide referrals to additional services. Risk factors that may indicate the need for genetic counseling include the following:

• Pregnant person aged 35 years or older at the estimated date of delivery (EDD)
• Other biological parent’s age of 50 years or older
• Known carriers of genetic disorders
• Pregnant person’s history of a minimum of two pregnancy losses
• Biological parent of a child with congenital anomalies (stillbirth or live birth)
• Biological parent of a child with developmental delays or sensory disorders
• Family history of either biological parent that includes a relative with genetic or congenital anomalies, developmental delays, or sensory disorders
• Consanguinity or incest
• Exposure to teratogens during pregnancy (Centers for Disease Control and Prevention [CDC], 2022)

The ideal time for obstetric care providers to offer genetic counseling is before conception. Preconception screening for both medical and genetic disorders opens up a dialog between the prospective biological parents. Genetic counseling, based on a comprehensive family genetic history, provides accurate information on the risks and incidence of genetic disorders as a couple in order for the pregnant person to make the most informed decision. A list of genetic disorders and congenital anomalies to include when obtaining the genetic history is included in Table 10.12.

Several factors influence a person’s choice to undergo genetic counseling and testing. These factors include socioeconomic, cultural, and religious considerations. Legal, ethical, and moral considerations surrounding decisions based on genetic risks make the decision making a complex process. Nurses who provide information to parents and families about genetics must have the following competencies: interviewing skills, ensuring confidentiality and informed consent, and provision of ethical, legal, psychosocial, and culturally appropriate care. Knowledge competencies include inheritance patterns, genetic probability, financial aspects of counseling and laboratory testing, and the risks and benefits of genetic testing. It is also important for the nurse to know their limitations and refer patients and families to other individuals, groups, or agencies when necessary.

Embryonic Stage of Development

The embryonic stage of fetal development begins with the completion of implantation and ends at the start of week 9 of gestation. The weeks of gestation during pregnancy are calculated from the first day of the pregnant person’s last menstrual period rather than from the date of conception. In the final days of the pre-embryonic stage, the blastocyst develops into three germ layers called the ectoderm, mesoderm, and endoderm. The formation and development of the organs of the body, called organogenesis, starts at the beginning of the embryonic stage. The three germ layers develop into the organs, tissues, and structures of the body.

Although the nervous system starts forming first, the embryo’s cardiovascular system begins functioning first. The hemoglobin formed by the embryo has a higher affinity for oxygen than adult hemoglobin because the embryo is dependent on the parent for oxygen. The embryo also requires more oxygen to support its rapid growth. The nervous system works within the first 8 weeks of gestation but is not fully developed at birth. This is evident by a newborn’s lack of neuromuscular coordination, developed senses, or speech. The brain is not fully developed until 5 years of age. In most cases, the gonads differentiate into male or female by 8 weeks. The testes stay in the inguinal canal, fully descending between 34 and 36 weeks of gestation.

Exposure to teratogens during organogenesis leads to an increase in the risk of malformations in all systems of the embryo’s body (Figure 10.15). During organogenesis, the trachea and esophagus are one tube. The separation into esophagus and trachea is not complete until 5 weeks of gestation. If the pregnant person is exposed to a teratogen at this critical point, then a congenital anomaly called a tracheal-esophageal (TE) fistula could result. In this anomaly, the esophagus is attached to the trachea rather than to the stomach. Table 10.13 summarizes the weekly changes in embryo development.

Figure 10.15 Fetal Development This chart shows the development of the organs, structures, and tissues of the embryo and fetus and the week of gestation each organ or structure is most vulnerable to teratogens. *This fetal chart shows 38 weeks of pregnancy. Since it is difficult to know exactly when conception occurs, health care providers calculate a woman's due date 40 weeks from the start of her last menstrual cycle. (credit: “Fetal development chart” by CDC, Public Domain)

Fetal Stage of Development

The fetal stage of development begins at 9 weeks of gestation and ends with birth (Figure 10.16). Organs, tissues, and structures of the fetus continue to develop and begin to function. The fetus has its own circulation, and the umbilical cord connects the fetus to the placenta. The skin on the fetal abdomen closes at around 10 weeks of gestation. Exposure to teratogens at this part of organogenesis can result in abdominal congenital anomalies known as gastroschisis (failure of the upper abdomen to close, allowing the stomach to protrude) or omphalocele (failure of the abdomen to close at the umbilicus, allowing the intestines to protrude).

Figure 10.16 Early Fetal Development At 9 weeks, the fetus can move its arms and legs, the heart is formed, the brain is developing, and the intestines are outside the abdomen. (credit: “9-Week Human Embryo from Ectopic Pregnancy (7th week p.o.)” by Ed Uthman/Wikimedia Commons, CC BY 2.0)

Skeletal system development reaches an important step when ossification starts at 12 weeks’ gestation. The kidneys are capable of producing urine at 12 weeks. Urine production influences the amount of amniotic fluid that is present. Urine production is also required for fetal lung development. When urine is not excreted into the amniotic fluid, as in the case of Potter syndrome, the lungs remain hypoplastic, incapable of inflating. At 16 weeks, the fetus starts forming stool called meconium. Unless infection is present, the fetal gut remains sterile until after birth. Digestive enzymes are not secreted sufficiently until 36 weeks of gestation. This is why preterm newborns do not tolerate formula or even breast milk well. The lungs are among the last organs to complete anatomic development. Until 24 weeks of gestation, the alveoli are not formed or present in sufficient numbers. Without the surface area in the alveoli for gas exchange in the lungs, the fetus has not reached viability, or the ability to survive outside the uterus. For the alveoli to expand and retract for respiration, the lungs need to produce surfactant, a mixture of fats and proteins produced in the lungs that coats the alveoli. When the newborn exhales, surfactant keeps the alveoli from sticking together, interfering with expansion of the alveoli during the next inhalation. The fetus starts producing surfactant at 24 weeks. Table 10.14 lists the milestones in fetal development by weeks of gestation.

Formation of the Placenta

The placenta is formed from a blend of tissue from the embryo and the pregnant person. The chorionic membrane and chorionic villi make up the fetal side of the placenta. The chorionic villi develop into the fetal blood vessels that will merge into the umbilical cord vessels. The fetal blood vessels protrude into the decidua basalis, the pregnant person’s side of the placenta. The decidua basalis is one of the three layers of the endometrium and helps to form the cotyledons, or lobes on the pregnant person’s side of the placenta (Figure 10.17).

Figure 10.17 Placenta The structures of the placenta allow for the exchange of nutrients and oxygen from the pregnant person and carbon dioxide and wastes from the fetus. (modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0)

The placental membrane separates the fetal side of the placenta from the pregnant person’s side. The placental membrane allows transfer of oxygen and nutrients to the fetus and carbon dioxide and waste products to the pregnant person, without the mixing of fetal and parental blood. The amnionic membrane, formed at the same time as the chorion, is part of the placenta and lies over the chorion. The amniotic membrane forms the sac around the embryo and fills with amniotic fluid as the fetus grows and develops. The formation of the placenta is usually complete by week 12 of gestation.

Functions of the Placenta, Membranes, and Amniotic Fluid

The placenta performs physiologic functions for the developing embryo and fetus. Respiration occurs in the placenta. Oxygen diffuses across the placenta membrane from the pregnant person to the fetus, and carbon dioxide diffuses across the placenta membrane from the fetus to the pregnant person. Nutritional intake is another physiologic function of the placenta. Glucose diffuses from the pregnant person to the fetus to provide energy. Amino acids cross the placenta membrane from the pregnant person to the fetus via active transport to be synthesized by the fetus for growth and development. Fatty acids are transported to the fetus via simple diffusion and are essential to building the brain and nerves. Water and electrolytes are transferred from the pregnant person to the fetus via passive diffusion. Iron, calcium, and vitamins require active transport across the placenta membrane from the pregnant person to the fetus. The placenta also acts as the kidneys for the fetus, transporting metabolic waste like urea, uric acid, and bilirubin to the pregnant person to eliminate.

Another important function of the placenta is the production of hormones to support the pregnancy and fetal growth and development. The placenta produces progesterone, estrogen, human chorionic gonadotropin (hCG), human placental lactogen (hPL), and relaxin.

• Progesterone serves two major functions: to facilitate implantation and to decrease uterine contractions.
• Estrogen stimulates enlargement of the uterus and breasts.
• Human chorionic gonadotropin supports the production of estrogen and progesterone from the corpus luteum until the placenta can produce these hormones.
• Human placental lactogen promotes fetal growth and parental breast development.
• Relaxin is produced first by the corpus luteum and then by the placenta. Relaxin increases the elasticity of the ligaments in the pregnant person, loosening the joints in the body. Relaxin helps prepare the pelvis to accommodate the fetus during the birth process.

The fetal membranes consist of the chorion and amnion. The functions of the fetal membranes are to contain the amniotic fluid and to help protect the fetus from infections. The functions of the amniotic fluid include protecting the fetus from injury, allowing freedom of movement for normal development of the fetus, and maintaining a consistent intrauterine temperature. The fluid is produced by the amniotic membrane in the first 14 weeks of the pregnancy and by the fetal kidneys during the remainder of the pregnancy.