The rapidly developing fetus depends on the mother for nutrients, oxygen, and waste elimination as substances pass through the placenta (a disk-shaped organ attached to the wall of the uterus, shown in I Figure 11.5). The fetus is joined to the placenta by the umbilical cord. The fetal blood circulates independently within the closed system of the fetus and the inner part of the placenta. Maternal blood flows in the uterine walls and through the outer part of the placenta. Fetal blood and maternal blood do not normally intermingle. All exchanges between the fetal and maternal circulatory systems
CHAPTER 11
occur by passage of substances through the walls of the blood vessels. Nutrients and oxygen from the maternal blood pass into the fetal blood vessels; carbon dioxide and waste products from the fetus pass into the maternal blood vessels, to be removed by maternal circulation.
The placenta prevents some kinds of bacteria and viruses—but not all—from passing into the fetal circulatory system. Many bacteria and viruses, including HIV, do cross through the placenta. Certain prescription medications, legal drugs such as tobacco and alcohol, and illegal drugs are dangerous to the developing fetus. Tobacco and alcohol affect a far greater number of babies each year, with more significant health consequences, than do illegal drugs (Yuan et al., 2001).
Approximately 13% of U. S. women continue to smoke throughout their pregnancies; smoking reduces the amount of oxygen in the bloodstream and thereby increases the chances of miscarriage, low fetal birth weight, and other pregnancy complications that can result in fetal or infant death (Fifer et al., 2009). Children of mothers who smoked during pregnancy have more respiratory diseases, have a 50-70% greater chance of having a cleft lip or palate, and have significantly lower developmental scores and an increased incidence of reading disorders compared with matched children of nonsmokers (Williams, 2000). Research in Denmark found that prenatal tobacco exposure was correlated with hormonal changes in adolescence and adulthood. Males had smaller testicles, lower total sperm counts, impairment in testicular function, lower adult height, and a higher adult fat to body size ratio (Ravnborg et al., 2011). Daughters of women who smoked 10 or more cigarettes a day began menstruating almost three months earlier than daughters of nonsmoking mothers (Shrestha et al., 2011).
(a)
I Figure 11.5 The placenta exchanges nutrients, oxygen, and waste products between the maternal and fetal circulatory systems. (a) the placenta attached to the uterine wall. (b) Close-up detail of the placenta.
Conceiving Children: process and Choice
Alcohol easily crosses the placental membranes into all fetal tissues, especially brain tissue. Fetal alcohol syndrome (FAS) is the leading cause of birth defects and developmental disabilities in the United States. Because a safe level of alcohol consumption is unknown, the Food and Drug Administration has advised women to abstain completely from alcohol use during pregnancy to avoid the risk of damage to their babies. One drink per day has been associated with adverse birth effects, and binge drinking (five or more drinks per occasion) is extremely toxic to the fetus. Alcohol use can cause intrauterine death and spontaneous abortion, premature birth, congenital heart defects, damage to the brain and nervous system, and numerous physical malformations of the fetus. The effects of FAS persist through childhood; children with FAS continue to be smaller than normal and developmentally delayed and to have behavior problems (Will — ford et al., 2006). Perhaps as a result of public health educational efforts, fewer women drink alcohol during pregnancy: In 2004, 12% of pregnant women drank alcohol, compared to 30% in 1989 (Grant et al., 2009).
The babies of mothers who regularly used illegal addictive drugs, such as amphetamines and opiates, during pregnancy are often born premature and have low birth weight. In addition, after birth these babies experience withdrawal from the drug: They have tremors, disturbed feeding and sleep patterns, and abnormal muscle tension, and they often require hospitalization in neonatal intensive care units. These children can experience permanent birth defects and damage to sensory, motor, and cognitive abilities that continue past infancy (Zambrana & Scrimshaw, 1997).
In a number of tragic situations, children have been damaged by prescription and over-the-counter medications taken by their mothers during pregnancy. For example, the drug thalidomide, prescribed as a sedative to pregnant women in the early 1960s, caused severe deformities to the extremities. Some grown children of women who were given diethylstilbestrol (DES) while pregnant have developed genital tract abnormalities, including cancer (Koren, 2009). Antibiotics need to be prescribed selectively during pregnancy because tetracycline, a frequently used antibiotic, can damage an infant’s teeth and cause stunted bone growth if it is taken after the 14th week of pregnancy. Many over-the-counter medications, such as ibuprofen, aspirin, and histamines, can be detrimental to the fetus, and effects are unknown for many other nonprescription drugs and herbs. A woman and her health-care provider should carefully evaluate medications used during pregnancy (Mitchell et al., 2011).