10.5: Fetal Growth and Development
By the end of this section, you will be able to:
- Explain the relevance of genetics in conception and fetal development
- Differentiate between the stages of fetal development
- Explain the physiology of the placenta
- Explain fetal circulation
- Discuss important milestones of fetal growth and development
- Explain the effects of important influences on fetal growth and development
When providing prenatal care, the nurse should understand the basic laws of genetic inheritance in order to provide preconceptual counseling and basic genetic information. Knowledge of the processes of conception and embryonic and fetal development is important for the nurse to understand why and when the fetus is vulnerable. The nurse also needs to comprehend how lifestyle behaviors and environmental exposures affect the pregnancy and the growth and development of the embryo and fetus. Understanding the laws of inheritance, fetal growth and development, and the multiple influences on fetal growth and development is part of the foundation of prenatal care.
Genetics
The study of heredity and the patterns of inherited traits is called genetics . The basics of genetics include understanding the role of chromosomes and genes. Chromosomes are found in the nucleus of every cell in the human body and contain all the person’s genes. Genes are short strands of deoxyribonucleic acid (DNA) found within the chromosome; they carry the traits of inheritance and direct many functions to maintain the health of the human body. The science of genomics maps and studies the functions of genomes, the gene sequencing forming the DNA chain within any cell in the body. Each gene instructs a specific set of proteins to perform a specific function within a person’s body. Any change in the structure or location of a gene within a DNA molecule results in an abnormality within a specific function of the body, such as lactose intolerance. Advances in genetics and genomics contribute to the growing knowledge of prevention, detection, and diagnosis of disease and provide health care providers the means to develop personalized treatment plans. For a list of disorders caused by genetic mutations, see Table 10.11.
| Disorder | Description |
|---|---|
| Sickle cell disease | Changes the structure of the red blood cell, diminishing the ability of the RBC to transport oxygen; most commonly found genetic disorder in people of African descent (recessive) |
| Cystic fibrosis | Produces thick mucus that blocks the bronchi and fibrous tumors causing pancreatic insufficiency; most commonly found genetic disorder in people of European descent (recessive) |
| Tay-Sachs disease | Degenerative nervous system disease resulting in the death of the infant by 2 years of age; most commonly found genetic disorder in people of Ashkenazi Jewish descent (recessive) |
| Huntington chorea | Progressive neuromuscular disorder affecting motor, memory, and behavior (dominant) |
| Hemophilia | Lack of factor VIII in the clotting cascade; without replacement factor VIII, bleeding cannot be controlled (X-linked) |
Perinatally, genetics and genomics present facts and risks to the expectant parents and their families regarding inherited disorders or anomalies. Some families are faced with decisions before they conceive, such as when both prospective parents know they carry the recessive sickle cell trait. Other families make decisions after conception when informed the fetus has a chromosome or genetic disorder, such as Down syndrome or cystic fibrosis .
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.
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.
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.
What do you know about birth defects? Take this quiz from the University of Rochester Medical Center to test your knowledge.
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.
| Genetic Disorders | Congenital Anomalies |
|---|---|
|
Thalassemia
Tay-Sachs disease Sickle cell anemia Cystic fibrosis Hemophilia Muscular dystrophy Spinal muscular atrophy Neurofibromatosis Huntington disease Epilepsy Phenylketonuria (PKU) Chromosome abnormality |
Congenital heart disease
Cleft lip and/or cleft palate Limb defects Deafness Blindness Neural tube defects Developmental delay Unexplained pregnancy loss |
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.
Nurse:
L.K.
Clinical Setting:
Labor and delivery unit
Geographic Location:
South Carolina
I remember when a patient came into the labor and delivery unit with a known fetal congenital anomaly that was incompatible with life outside the uterus . The anomaly was anencephaly (a serious birth defect in which a baby is born without parts of the brain and skull). I was assigned to the patient, who was in early labor. During the admission process, I asked the patient what her plans were for pain relief. The patient stated an epidural and spontaneously said she did not want her mind clouded with drugs so she would be alert during Angel’s (the name the couple had given to their baby) birth and transition. The patient’s husband volunteered that they both had had genetic counseling , but the most important influence on their decision to continue the pregnancy was their minister. The minister also went to genetic counseling to become more informed about the anomaly and the prognosis for the fetus. While in early labor, the patient and her husband openly discussed how they arrived at the decision of continuing to provide life to the fetus and newborn for however long the newborn survived, inside or outside the uterus.
I did not ask this couple to explain their decision-making process. The information was freely given to me. I was not present for the birth of Angel, but I was able to realize how the information this couple received during the genetic counseling played a role in both continuing the pregnancy and the birth plan.
Fetal Development
Nurses practicing in perinatal care need to understand and be able to explain the processes of conception and embryonic and fetal development . Conception, or fertilization, is when the sperm and ovum unite to form the zygote and typically occurs within the fallopian tube. After the zygote has divided into 16 cells, the zygote becomes the morula . Peristalsis and cilia within the fallopian tube help to move the morula into the uterus. The time from conception to when the morula enters the uterus is about 72 hours. Over the next several days, the cells of the morula divide into specialized cells, blastocyst and trophoblast , that will create the fetal structures. The blastocyst is the group of cells forming the embryo and the amnion. The trophoblast is the group of cells forming the placenta and the chorion. The trophoblast and blastocyst implant in the endometrium within 7 to 10 days after conception. When the trophoblast has successfully attached to the endometrium, implantation occurs. The days from conception to implantation are the pre-embryonic stage of development (Figure 10.14).
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.
| Week of Gestation | Structure and System Development |
|---|---|
| Week 3 |
Neural tube: brain and spinal cord
Heart and gastrointestinal tract Limb buds form |
| Week 4 |
Brain structures begin to form
Stomach, pancreas, and liver start to form Limb buds enlarge |
| Week 5 |
Cranial nerves start to form
Heartbeat is detectable Neuromuscular connections are establishing Eyes and ears are beginning to form |
| Week 6 |
CNS forms, and brain waves are detectable
Fetal circulation is established RBCs are starting to be formed in the liver Lungs begin forming Skeletal structure is laid out |
| Week 7 |
Arm and leg movements, toes visible
Mouth and lips formed Diaphragm formed Nipples, hair follicles, and genitals form |
| Week 8 |
Heart is developed
Facial features continue to develop Intestinal rotation occurs Bone cells replace cartilage |
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).
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.
| Week of Gestation | Structure and System Development |
|---|---|
| Week 12 |
Ossification begins at 12 weeks and continues through childhood
Urine is formed Limbs move at the joints Eyelids are fused Palate is completely formed Fetal heart rate can be heard by Doppler |
| Week 16 |
Pinna of ear is formed
Respiratory system is formed Hand and foot ridges are formed Fetus is able to make sucking motions Meconium is forming in the intestines |
| Week 20 |
Pancreas is producing insulin
Sleep and activity pattern is present Lanugo and vernix cover the body Nails are formed Brown fat begins to develop |
| Week 24 |
Now viable because alveoli in the lungs are forming in preparation for breathing, and surfactant is starting to be produced by the alveoli
Eyelids begin to open Capable of hearing |
| Week 28 |
Eyelids open and close
Blood formation now occurs in bone marrow Nervous system takes over more functions Fetus has a sleep-wake cycle Fetus reacts more to the sound of the pregnant person’s voice |
| Week 32 |
Bones are developed
Increase in the number of adipose cells Breathing movements are rhythmic |
| Week 37 |
Enzymes for digestion not secreted efficiently until 36 weeks
Testes do not descend until 34–36 weeks Fetus is considered full term at 37 weeks |
You can watch video footage of fetal development inside the womb to observe the stages of development.
Formation and Functions of the Placenta
The placenta is a unique organ because it develops for the sole purpose of providing nutrients and oxygen to the developing embryo and fetus and removing the fetal waste products and carbon dioxide. The placenta has a limited lifespan, ending with its delivery after the birth of the newborn. The formation of the placenta starts at implantation .
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).
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.
Fetal Circulation
Fetal circulation allows oxygenated blood from the placenta to travel to the major functioning organs of the fetus, and it allows deoxygenated blood to go back to the placenta. This flow of blood requires a different circulation pattern in the fetus than is present in the newborn. The umbilical cord usually inserts into the center of the placenta and contains two arteries and one vein. Oxygenated blood is transported to the fetus from the placenta via the umbilical vein. The ductus venosus connects the umbilical vein to the inferior vena cava of the fetus. This places the oxygenated blood near the right atrium. The foramen ovale shunts the oxygenated blood from the right to left atrium, allowing some of the blood to bypass the right ventricle. The ductus arteriosus connects with the pulmonary artery to the descending aorta, bypassing the lungs so that more oxygenated blood is circulated throughout the body of the fetus. The deoxygenated blood is carried back to the placenta via the umbilical arteries that branch off the right and left internal iliac arteries (Figure 10.18). Fetal circulation is necessary, since the lungs are filled with fluid and cannot supply oxygen to or remove carbon dioxide from the fetus until after birth. When the newborn takes their first breath and starts crying, the pressure within the lungs causes the foramen ovale to close. Within 24 to 48 hours, the ductus arteriosus closes as well because the blood is now circulating in the pulmonary system. When the umbilical cord is clamped and blood ceases to flow through the ductus venosus, it collapses, closing as well.
Influences on Fetal Development
The nutrition and lifestyle habits of the pregnant person, environmental hazards, teratogens, TORCH and other infections, and maternal preexisting conditions can all affect the growth and development of the embryo and fetus. Embryonic and fetal development are affected by the pregnant person’s nutrition prior to and during pregnancy . A deficiency in folic acid has been linked to neural tube defects ( spina bifida , myelomeningocele , encephalocele , and anencephaly ). Too much vitamin A, as from the acne treatment isotretinoin (Accutane) , is linked to cleft lip and palate. Maternal exposure to environmental hazards can be through the lungs, ingestion, and exposure to an organism. A teratogen is anything known to affect the normal growth and development of the embryo or fetus. Teratogens include chemicals, drugs, and lifestyle habits.
TORCH , a group of infectious diseases that can be passed from the pregnant person to the fetus, is a known teratogen. The diseases included in the group are
- toxoplasmosis,
- other (originally only syphilis),
- rubella,
- cytomegalovirus, and
- herpes.
The causative viruses, bacteria, or organisms are capable being transported through the placenta membrane and affecting multiple fetal organ systems, specifically the brain, heart , eyes, ears, and skin. Table 10.15 lists TORCH infection effects on the fetus. If syphilis goes untreated or is treated in the second or third trimester, the newborn will have congenital syphilis or be stillborn. Hepatitis most commonly causes liver disease in the newborn.
| Infection | Effect on the Fetus |
|---|---|
|
Toxoplasmosis
protozoan |
Spontaneous abortion, low birth weight (LBW), hepatomegaly, neurologic damage |
|
Other: Hepatitis B
virus |
Chronically infected, HBV carrier, liver disease |
|
Other: Syphilis
bacteria |
Neurologic damage, congenital syphilis, stillbirth |
|
Rubella
virus |
Congenital rubella syndrome
Deafness, eye anomalies, neurologic deficits, cardiac anomalies |
|
Cytomegalovirus
herpes group virus |
LBW, hearing impairment, microcephaly, neurologic deficits |
|
Herpes
chronic viral infection |
Neurologic deficits, hepatic encephalopathy |
If a pregnant person is human immunodeficiency virus ( HIV ) positive and their CD4 count does not fall below 400, the infant will usually convert to seronegative within 6 months to 2 years of age if the person takes zidovudine (AZT) during pregnancy and/or during labor and birth. Additional information on teratogens and other influences on the growth and development of the fetus are discussed in Chapter 12 Pregnancy at Risk.