radiation, air pollution, and extremes of heat and humidity can impair development. Infants prenatally exposed to heavy metals, such as lead and mercury, whether through ingestion or inhalation, score lower on tests of cognitive ability and intelligence and have higher rates of childhood illness (Sadler, 2018; Vigeh, Yokoyama, Matsukawa, Shinohara, & Ohtani, 2014; Xie et al., 2013). Exposure to radiation can cause genetic mutations. Infants born to mothers pregnant during the atomic bomb explosions in Hiroshima and Nagasaki and after the nuclear power accident at Chernobyl displayed many physical deformities, mutations, and intellectual deficits. Prenatal exposure to radiation is associated with Down syndrome, reduced head circumference, intellectual disability, reduced intelligence scores and school performance, and heighted risk for cancer (Chang, Lasley, Das, Mendonca, & Dynlacht, 2014). About 85% of the world’s birth defects occur in developing countries, supporting the role of context in influencing prenatal development directly via environmental hazards but also indirectly through the opportunities and resources for education, health, and financial support (Weinhold, 2009).
Maternal Characteristics
Teratogens—and the avoidance of them—are, of course, not the only determinants of how healthy a baby will be. A pregnant woman’s characteristics, such as her age and her behavior during pregnancy, including nutrition and emotional well-being, also influence prenatal outcomes.
Good nutrition promotes healthy prenatal development.
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Nutrition
Nutrition plays a role in prenatal development both before and after conception. The quality of men’s and women’s diets influences the health of the sperm and egg (Sinclair & Watkins, 2013). Most women need to consume 2,200 to 2,900 calories per day to sustain a pregnancy (Kaiser, Allen, & American Dietetic Association, 2008). Yet about 41 million people in the United States (about 12% of households) reported food insecurity in 2016 (U.S. Department of Agriculture, 2017). That is, at least sometimes they lacked access to enough food for an active healthy lifestyle for all members of the household. About 795 million people of the 7.3 billion people in the world, or one in nine, suffer from chronic undernourishment, almost all of whom live in developing countries (World Hunger Education Service, 2017). Fetal malnutrition is associated with increased susceptibility to complex diseases in postnatal life (Chmurzynska, 2010). Dietary supplements can reduce many of the problems caused by maternal malnourishment, and infants who are malnourished can overcome some of the negative effects if they are raised in enriched environments. However, most children who are malnourished before birth remain malnourished; few have the opportunity to be raised in enriched environments after birth.
Some deficits resulting from an inadequate diet cannot be remedied. For example, inadequate consumption of folic acid (a B vitamin) very early in pregnancy can result in the formation of neural tube defects stemming from the failure of the neural tube to close. Spina bifida occurs when the lower part of the neural tube fails to close and spinal nerves begin to grow outside of the vertebrae, often resulting in paralysis. Surgery must be performed before or shortly after birth, but lost capacities cannot be restored (Adzick, 2013). Spina bifida is often accompanied by malformations in brain development and impaired cognitive development (Donnan et al., 2017). Another neural tube defect, anencephaly, occurs when the top part of the neural tube fails to close and all or part of the brain fails to develop, resulting in death shortly after birth. As researchers have learned and disseminated the knowledge that folic acid helps prevent these defects, the frequency of neural tube defects has declined to about 1 in 1,000 births (Viswanathan et al., 2017; Williams et al., 2015). However, in a national study of U.S. mothers, only 24% consumed the recommended dose of folic acid during pregnancy (Tinker, Cogswell, Devine, & Berry, 2010).
Emotional Well-Being
Although stress is inherently part of almost everyone’s life, exposure to chronic and severe stress during pregnancy poses risks, including low birthweight, premature birth, and a longer postpartum hospital stay (Field, 2011; Schetter & Tanner, 2012). Maternal stress influences prenatal development because stress hormones cross the placenta, raising the fetus’s heart rate and activity level. Long-term exposure to stress hormones in utero is associated with higher levels of stress hormones in newborns (Kapoor, Lubach, Ziegler, & Coe, 2016). As a result, the newborn may be more irritable and active than a low-stress infant and may have difficulties in sleep, digestion, and self-regulation (Davis, Glynn, Waffarn, & Sandman, 2011; Kingston, Tough, & Whitfield, 2012). Later in childhood, he or she may have symptoms of anxiety, attention-deficit/hyperactivity disorder, and aggression (Glover, 2011). Prenatal stress may also have epigenetic effects on development, influencing stress responses throughout the lifespan (Van den Bergh et al., 2017). Stress in the home may make it difficult for parents to respond with warmth and sensitivity to an irritable infant (Crnic & Ross, 2017). Social support can mitigate the effects of stress on pregnancy and infant care (Feldman, Dunkel-Schetter, Sandman, & Wadhwa, 2000; Ghosh, Wilhelm, Dunkel-Schetter, Lombardi, & Ritz, 2010).
Maternal Age
U.S. women are becoming pregnant at later ages than ever before. As shown in Figure 3.4, since 1990, the pregnancy rate has increased for women ages 35 to 39 and 40 to 44 and decreased slightly for women in their 20s (Hamilton et al., 2017). Does maternal age matter? Women who give birth over the age of 35, and especially over 40, are at greater risk for pregnancy and birth complications, including miscarriage and stillbirth, than are younger women. They are more vulnerable to pregnancy-related illnesses such as hypertension and diabetes, and their pregnancies involve increased risks to the newborn, including low birthweight, preterm birth, respiratory problems, and related conditions requiring intensive neonatal care (Frederiksen et al., 2018; Grotegut et al., 2014; Kenny et al., 2013; Khalil, Syngelaki, Maiz, Zinevich, & Nicolaides, 2013). The risk of having a child with Down syndrome also increases sharply with maternal age, especially after age 40 (Diamandopoulos & Green, 2018; Hazlett, Hammer, Hooper, & Kamphaus, 2011) (see Figure 3.5).
Figure 3.4 Birthrates by Age of Mother, United States, 2017
Source: J. A. Martin et al. (2018).
Figure 3.5 Maternal Age and Down Syndrome
Source: Data are from Mai et al. (2015).
Although risks for complications rise linearly with each year (Yaniv et al., 2011), it is important to realize that the majority of women over age 35 give birth to healthy infants. Differences in context and behavior may compensate for some of the risks of advanced maternal age. For example, longer use of oral contraceptives is associated with a lower risk of giving birth to a child with Down syndrome (Nagy, Győrffy, Nagy, & Rigó, 2013).
Paternal Characteristics
It is easy to see how mothers’ health, behavior, and contexts influence prenatal development, but what about fathers? It was once thought that fathers had no influence on prenatal development and researchers thus neglected to study the father’s role. Most obviously, fathers influence the home context. Second-hand smoke from fathers is harmful to the developing organism (Wang et al., 2018). Fathers’ interactions with pregnant mothers can increase maternal stress, with potential negative implications for prenatal development. We know less about how fathers’ health, behaviors, and contextual factors act as biological influences on prenatal development.
Advanced paternal age is associated with an increased risk of birth