2.3: Pregnancy and Prenatal Development

Learning Objectives

By the end of this section, you will be able to:

• Describe the major periods of prenatal development
• Identify methods used to monitor fetal development
• Identify sources of environmental influence on the embryo/fetus during prenatal development
• Describe the role of parents during pregnancy

When Olivia learned in August that they were pregnant, they joyfully anticipated the arrival of a baby the following April and hoped the baby might share their late father’s birthday. Olivia didn’t know the exact date of conception, because they had been trying to conceive for several months, so the obstetrician used the first day of the last menstrual period and added two weeks, given that ovulation typically occurs midcycle. However, Olivia's doctor warned her that pregnancies can vary in length, and that very few births occur on the baby’s due date (Declercq et al., 2023).

Pregnancies are often divided into three-month periods called trimesters. However, three developmental periods also occur, each marked by major changes and occurring on a different schedule than the trimesters (Figure 2.15). Each developmental period of pregnancy involves important aspects of maternal and paternal health that can influence the health of the pregnancy, environmental factors that can affect the health of an embryo or fetus during pregnancy, and education and behaviors parents can optimize to ensure a healthy pregnancy.

Figure 2.15 While pregnancy is often discussed in terms of trimesters, the developing fertilized egg goes through its own three transformative stages: the germinal, embryonic, and fetal stages. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license; credit left: modification of work “Sperm-egg” by PD Images/Wikimedia Commons, Public Domain; credit middle: modification of work “Tubal Pregnancy with embryo” by Ed Uthman/Wikimedia Commons, Public Domain)

Prenatal Development and Viability

The three major developmental periods of pregnancy are the germinal period, the embryonic period, and the fetal period. Significant development changes occur during all of these stages, and during the last, the fetus becomes viable outside the womb. Complications, such as exposure to teratogens and congenital disorders, may occur during fetal development.

The Germinal Period

As you’ve learned, conception occurs when a sperm cell enters the ovum and the twenty-three chromosomes in each of the two cells combine, creating a unique new combination of forty-six chromosomes. The single cell formed at conception is called a zygote.

The first and shortest of the three developmental periods of pregnancy is the germinal period. It starts at conception and lasts approximately two weeks. The zygote starts to divide within twenty-four hours of forming, and around five to seven days later it is a small cluster of a few hundred cells called a blastocyst. About a week after conception, the blastocyst starts to form burrlike projections, which help it implant in the uterus after it travels down the fallopian tube. Cell differentiation starts during the germinal period as well.

Cell differentiation is the process in which stem cells, guided by genes, start to specialize or take on characteristics of what they will later become, such as organs, nails, hair, or skin. This is the start of the blastocyst’s transformation into an embryo. One end of the blastocyst turns into the embryoblast, which will eventually become the embryo. The rest develops into a life-support system for the embryo consisting of the placenta, umbilical cord, amnion, and chorion.

The germinal period is also the time during which twins and other multiples are conceived (Figure 2.16). Fraternal or dizygotic twins develop when two ova are fertilized at the same time. In the case of identical or monozygotic twins, for reasons still not completely understood, the zygote splits into two clusters of cells, creating two genetically identical zygotes (Hall, 2021). In pregnancies that are not due to fertility treatments, there is a 0.4 percent chance of having monozygotic twins (Roberts et al., 2018). That chance increases to a 1.57 to 5.6 percent chance in pregnancies utilizing IVF and other fertility treatments, possibly due to medications and laboratory procedures employed in the IVF process (Roberts et al., 2018).

Figure 2.16 In the development of identical twins, a zygote splits into two separate but genetically identical zygotes. In the case of fraternal twins, two eggs are fertilized by two separate sperm at the same time. (credit: modification of work “Identical twins lg” by National Human Genome Research Institute/Wikimedia Commons, Public Domain)

By the end of the germinal period, the blastocyst has traveled through the fallopian tube to the uterus, where it implants into the uterine wall. This process is called implantation. Successful implantation is not guaranteed, and it’s estimated that 15 percent of pregnancies are lost before or during this process (Jarvis, 2020). It is difficult to develop methods to correctly assess the success of implantation without knowing the exact date of conception.

The Embryonic Period

Once implantation has occurred, the blastocyst becomes an embryo, and the embryonic period begins. This period lasts from the third to the eighth week of pregnancy. An enormous number of changes happen during this period. After implantation, part of the embryo rapidly forms four major support structures—the amnion, chorion, placenta, and umbilical cord:

• The amnion or amniotic sac is a watertight sac that protects the embryo and is filled with amniotic fluid.
• The chorion gathers nourishment for the embryo and eventually becomes the lining of the placenta.
• The placenta is a temporary organ connecting the uterus to the umbilical cord that provides respiration and nourishment for the embryo in addition to eliminating metabolic wastes.
• The umbilical cord is a flexible tube connecting the embryo and the placenta; it contains three types of blood vessels that carry nutrients and oxygen from the birth mother to the embryo and transport waste from the embryo to the placenta (Heil & Bordoni, 2022).

Other changes occurring during the embryonic period include organogenesis, or the formation of organs. Ongoing cell differentiation allows cells to become specialized to do different jobs. For example, liver cells help filter out toxins, neural cells help process information, and heart cells beat rhythmically to push blood through the embryonic body. This process begins approximately three weeks after conception, when the embryo starts to differentiate into three distinct components, the ectoderm, mesoderm, and endoderm. The outer layer of the embryo becomes the ectoderm, and cell differentiation allows it to start developing the nervous system, skin, and hair. The middle layer of the embryo, the mesoderm, develops into muscles, bones, and the circulatory system. The innermost layer of cells within the embryo becomes the endoderm and develops into the organs of the digestive system, the lungs, and the urinary tract.

Link to Learning

Watch this TED talk discussing prenatal development to learn more about how embryos develop.

The Fetal Period

The last seven months of pregnancy are a period of organ growth and refinement called the fetal period. During this time, the fetus grows quickly. Around seventeen to twenty weeks after conception, lanugo and vernix develop (Verhave & Lappin, 2018). Vernix is a white cheese-like substance that helps protect the skin from chafing in the amniotic fluid. Lanugo is a soft, thin hair that helps vernix stick to the skin of the fetus in addition to helping protect the skin itself. Both lanugo and vernix usually disappear between the thirty-third and thirty-sixth weeks of pregnancy (Verhave & Lappin, 2018).

Growth occurs in several directions both prenatally and after birth. Growth that occurs from head to feet is called cephalocaudal growth, whereas proximodistal growth occurs from the center of body outward. Cephalocaudal growth allows the fetus to grow longer, whereas proximodistal growth is exemplified by infants being able to control their arm movements before their fingers. In mass-to-specific growth, large movements and structures develop before smaller or more specified movements and structures.

The Age of Viability

The more time the fetus spends inside the womb, the greater its chance of survival. The age of viability is the age at which a fetus born prematurely (before the thirty-seventh week of pregnancy) may have a chance of survival outside the womb, usually with intensive medical intervention (Jang & Lee, 2022). In the United States, the earliest age of viability is generally considered to be the twenty-fifth week of pregnancy. However, the age of viability has changed over the years and is not the same from country to country. Medical systems and resources vary from place to place. Not all neonatal intensive care units (NICUs) can handle babies born at or before twenty-five weeks of gestation. Extremely premature babies require extensive medical care to survive and may also have long-term cognitive and/or physical issues.

The earliest known surviving infant was born at twenty-one weeks’ gestation; others have survived after being born at twenty-two weeks’ gestation. All required intensive medical treatment to survive because several important systems were not sufficiently developed, including the lungs (American College of Obstetricians and Gynecologists, 2023). Babies born near the lower limits of the age of viability have an increased risk of neurological and other health issues, including heart and lung problems later in life (Jobe, 2022). It is estimated that approximately 50 percent of babies born before twenty-four weeks’ gestation may survive, but none without intensive medical intervention (Thomas & Asztalos, 2021).

Complications in Prenatal Development

Sometimes issues develop during a pregnancy that can lead to spontaneous abortion, which is the loss of a pregnancy during the first twenty weeks. Spontaneous abortion, often called miscarriage, is the most common pregnancy complication and is estimated to occur in approximately 15 percent of all pregancies (Laisk et al., 2020). Of those lost pregnancies, about 60 percent terminate during the germinal period due to the embryo's failure to grow or implant properly. About 20 percent of those miscarriages occur between weeks six and ten of gestation, in the embryonic stage. About 60 percent of spontaneous abortions occur during the germinal period due to failure to grow or implant properly. About 20 percent occur between weeks six and ten of gestation, in the embryonic stage.

Spontaneous abortions during the germinal and embryonic stages are believed to often be due to chromosomal abnormalities. During the fetal stage, around 5 percent of pregnancies result in a spontaneous abortion or stillbirth, the death of the fetus after twenty-eight weeks. Other risk factors include advanced maternal age, history of past miscarriages, certain chronic diseases, and chronic stress (Alves et al., 2023).

Spontaneous abortion is most likely to occur for reasons outside the birth mother’s control. In the United States, low socioeconomic status and lack of access to health care are associated with an increased risk of miscarriage (Oliveira et al., 2020). A much lower risk of fetal loss, approximately 1.39 percent globally, exists for stillbirths (Hug et al., 2021). Global public health efforts, including by the WHO, have helped reduce this risk (Hug et al., 2021).

Link to Learning

Explore the March of Dimes website on miscarriage that describes different types of spontaneous abortions, reasons they may occur, and warning signs of possible pregnancy loss.

Teratogens

Any disease, drug, or other environmental agent that can harm the embryo or the fetus physically or influence health or behavior after birth is called a teratogen. Generally, the influence of a teratogen on a body part is strongest when that part is developing, such as during organogenesis (Figure 2.17). After organs have formed, they are usually less vulnerable to teratogens, except for the nervous system, which always susceptible. Limb formation and other aspects of prenatal development show varied critical and sensitive periods for risk of harm from teratogens.

Figure 2.17 Prenatal development includes developmental periods during which different features of growth are sensitive to teratogens. (credit: modification of work “Figure 1. Sensitivity to teratogens during pregnancy” by Mehmet Semih Demirtaş/IntechOpen, CC BY 3.0)

The potential impact of a teratogen varies based on a range of factors including the specific teratogen, the length and amount of exposure, genetic susceptibility, and the quality of the prenatal environment. The longer the exposure or the higher the dose of a teratogen, the greater the likelihood of it doing harm. Some teratogens can cause a range of developmental issues, and in some cases, different teratogens contribute to s specific disorder. Embryos and fetuses can be harmed by teratogens to which their biological fathers were exposed, as well as by those to which they are exposed via the birth mother’s body during gestation (Table 2.5).

The long-term effects of some teratogens often depend on the quality of the postnatal environment. For instance, maternal exposure to pollution is related to low birth weight, preterm birth, and neurological effects (Rani & Dhok, 2023). All these put a developing child at increased risk of challenges to overall health and quality of life.

Table 2.5 Long-Term Impacts of Various Teratogens (sources: CDC, 2024a; CDC, 2024b; Cestonaro et al., 2022; Dack et al., 2022; Giuliani et al., 2022; Honein et al., 2017; Kaleelullah & Garugula, 2021; Lin et al., 2023; Lusskin et al., 2018; Mactier & Hamilton, 2020; Manicklal, 2013; March of Dimes, 2021; Nesheim et al., 2012; Orsolini et al., 2017; Ross et al., 2015)

Teratogen	Possible Consequences of Exposure
Environmental Toxin
Radiation	Organ anomalies, slowed growth, or nervous system abnormalities depending on exposure timing
Lead	Increased risk for miscarriage, preterm birth, and low birth weight; damage to brain, kidneys, nervous system; learning or behavior problems
Mercury	Brain damage and hearing and vision problems
Polychlorinated biphenyls (PCBs)	Low birth weight and premature birth
Infectious Diseases
Rubella (German measles)	Malformation of organs or miscarriage if exposed during first three months of pregnancy
HIV	Transmission of HIV from the biological mother to infant; antiviral treatment during prenatal period drastically reduces transmission to less than 5 percent
Cytomegalovirus (CSV)	Central nervous system damage and hearing loss
Genital herpes	Blindness or death if exposed to active herpes lesions in the birth canal
Toxoplasmosis and listeria	Miscarriage, preterm birth, and fetal death, depending on exposure timing
Zika virus	Microencephaly, hearing and vision loss, and intellectual disability
COVID-19	Preterm birth and neurodevelopmental disorders
Syphilis	Premature birth, low birth weight, and fetal death; congenital diseases
Influenza	Spina bifida, cleft palate, and defects in the neural tube
Medications
Aspirin	Miscarriage if high doses are taken early in pregnancy; increased risk of bleeding if low doses are taken
Certain antibiotics (e.g., streptomycin)	Hearing loss and incomplete development of bones and teeth
Thalidomide	Very short or missing arms and legs and other congenital birth defects when taken during early pregnancy
Certain antiseizure medications	Developmental delays during infancy
Certain antipsychotic medications	Increased risk of heart defects
Certain antidepressant medications	Risk is not clear
Psychoactive Drugs
Cocaine	Low birth weight and placental abruption/separation
Opioids and amphetamine	Preterm birth, reduced growth, heart issues, cognitive and behavioral issues
Nicotine	Low birth weight, premature birth, stillbirth
Alcohol	Damage to developing organs; fetal alcohol spectrum disorder
Heroin	Addiction requiring medical care to overcome withdrawal symptoms; damage to the visual system; long-term behavioral consequences
Cannabis	Decreased growth; deficits in attention and long-term executive function; increased impulsivity
Excessive caffeine	Deficiencies in short-term memory, attention, and impulse control; other neurobehavioral and neurological effects

However, research shows that other factors associated with the home environment fostered resilience in preterm infants and had a positive influence on the children’s academic performance in grade school (Wouldes, 2022). Parents or caregivers who actively engage with their infants, provide stimulating toys, and spend time teaching skills such as language and counting increase the chances that preterm and normal-weight infants will do well in school (Vanes et al., 2021; Wouldes, 2022). Interactions including singing to infants, telling them stories, and playing with them provide cognitive stimulation important to learning language and learning in general, regardless of culture (Lansford, 2021).

Extreme conditions such as harsh environments or war zones, in which people may face food shortages and tremendous stress, can have a negative impact on pregnancy. Severe emotional stress during pregnancy, including job stress, household stress (such as living paycheck-to-paycheck), or relational stress (such as being a victim of domestic violence), leads to increased risk for miscarriage, premature birth, low birth weight, respiratory illnesses, and digestive problems (Zhang et al., 2023). Additionally, climate change may be a reason for increased exposure to stress for many people around the world, because it is believed to be increasing weather extremes such as flooding, wildfires, extreme heat, poor air quality, and restricted access to food and clean water (Ha, 2022; Olson & Metz, 2020). Working to lower stress and improve resilience factors, such as improving social support and prenatal care resources, can reduce vulnerability to these adverse effects and improve health outcomes for both the biological mother and the newborn (Zhang et al., 2023; Nolvi et al., 2022).

Males can also influence the health of the fetus if they have been exposed to certain toxins such as lead, radiation, or pesticides, or if they regularly use drugs or smoke. These factors can affect the quality and mobility of their sperm and therefore the health of the fetus during pregnancy (Meng & Groth, 2017). Secondhand smoke can lead to lower birth weights, particularly with the type of regular exposure that would occur from someone in the same house as the developing fetus (Khader et al., 2010). Likewise, intimate partner violence has adverse effects on the pregnant person including depression, anxiety, PTSD, and on the developing fetus (Agarwal et al., 2023).

Factors other than teratogens may put the health or even the life of the birth mother, the fetus, or both at risk. The most common causes of such high-risk pregnancies are maternal health problems like high blood pressure, diabetes, obesity, the presence of multiple fetuses (twins, triplets), and “advanced” maternal and paternal age (defined as over age thirty-five years).

Congenital Disorders

Genetics and the environment are factors that can contribute to a congenital disorder, or an abnormality present at birth. Common congenital disorders include heart defects, clubfoot, and cleft palate (CDC, 2024c). Approximately 3 percent of infants born within the United States have a congenital disorder, and disorders affecting the heart, lungs, or other vital organs account for nearly 20 percent of infant deaths (CDC, 2024c). Worldwide, congenital disorders affect approximately 3–6 percent of all pregnancies (World Birth Defects Day 2023: Global Efforts to Raise Awareness and Support Families | CDC, 2024d). The mortality rate due to congenital disorders worldwide was estimated to be about 7.6 percent in 2020 (Perin et al., 2023). The difference in rates between the United States and the rest of the world is likely due to differences in reporting data; infant mortality rates may be underestimated in many countries, especially those with lower incomes (Perin et al., 2023).

Monitoring Prenatal Development

Physicians use a variety of methods to monitor the development and health of the growing fetus. Not only do these allow us to observe the development of structures and organs, but some can detect the presence of chromosomal abnormalities:

• Ultrasounds use high-frequency sound waves to create an image of the embryo’s or fetus’s developing structures and organs and are a typical feature of prenatal care in the United States. Transvaginal ultrasounds have helped researchers and physicians learn more about the early stages of pregnancy than traditional abdominal ultrasound did.
• Maternal serum tests are blood tests typically given between the eleventh and thirteenth weeks of pregnancy to measures levels of certain proteins (Graves et al., 2002). Abnormal levels of these proteins suggest a higher chance of the fetus having a chromosomal abnormality.
• A test in which a needle is inserted through the mother’s abdomen to take a sample of the amniotic fluid surrounding the fetus is amniocentesis; it is done around the sixteenth week of pregnancy. The sample is then tested for signs of chromosomal disorders (such as Down syndrome), some genetic disorders (like cystic fibrosis), and birth defects (such as spina bifida). Amniocentesis is typically only done when medically recommended.
• During the medical procedure chorionic villus sampling (CVS), a small piece of the placenta is removed to test for chromosomal and genetic abnormalities or defects. If necessary, it is performed in the first trimester. Like amniocentesis, CVS carries risks, including infection, limb deficiency, and/or miscarriage (Olney et al., 1995).

Link to Learning

Watch this short video about the amniocentesis procedure to learn more about its use in assessing a fetus for chromosomal and genetic abnormalities before birth.

Behaviors for a Healthy Pregnancy

A healthy pregnancy is never guaranteed, though certain prenatal behaviors and choices can improve the odds of having a healthy and safe experience. The person planning to carry the pregnancy should strive to maintain a healthy diet with sufficient levels of the macronutrients and micronutrients that support a healthy pregnancy. Macronutrients, including carbohydrates, proteins, and fats, provide the energy, protein, and fatty acids needed, while micronutrients provide the vitamins and minerals (Mousa et al., 2019). For example, folate is a B vitamin particularly important to fetal development and readily available in leafy green vegetables. Finally, the person carrying the pregnancy can make sure they get enough sleep, exercise regularly, stay up to date on their vaccinations, and mitigate any potential health issues such as diabetes, drinking, and smoking.

Finding a trusted obstetrician/gynecologist and/or midwife is also important, and prenatal care should begin as soon as the pregnancy is confirmed. Birth mothers who do not receive regular prenatal care are at higher risk of having a baby with low birth weight (Thorsen et al., 2019). Low birth weight is one major indicator that a newborn is at risk of a variety of postnatal complications. Regardless of culture, maintaining health while pregnant and getting regular prenatal care decrease the chances of having a baby with low birth weight (Khan et al., 2019; Zhou et al., 2019).

During prenatal visits, the obstetrician will check the vital signs of the pregnant person and the fetus to help assess the health of both; the pregnant person’s weight is also noted to ensure appropriate weight gain during the pregnancy. This is generally 25 to 35 lb, depending on pre-pregnancy weight. The pregnant person will also be advised on diet during pregnancy, including which foods to avoid to limit potential exposure to toxins like Listeria (a bacterial parasite) and the number of extra calories needed to help support the pregnancy (about 300 calories more per day than before becoming pregnant) (American College of Obstetricians and Gynecologists, 2023). Getting enough exercise is also important. Someone who was very active before pregnancy can typically maintain that activity level; however, it’s advisable to consult a health professional if starting a new exercise routine after becoming pregnant. Avoiding excessive stress matters too, as does preparing for the home arrival of the baby.

Link to Learning

Explore this online tool about beneficial food choices for those who are pregnant or breastfeeding from the U.S. Department of Agriculture. This helps ensure proper nutrition at each stage of the pregnancy.

Link to Learning

Learn about some of the surprising ways pregnancy affects the body in this TED Talk.

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