The Effects of Tobacco Use During and After Pregnancy

by National Institute of Health

Alcohol and tobacco use during pregnancy have both been associated with a number of adverse effects on the growth, cognitive development, and behavior of the exposed child. Understanding the effects of prenatal tobacco exposure allows researchers to identify those characteristics that are uniquely related to tobacco and those that are affected by alcohol exposure. This research, along with studies on the effects of alcohol use during pregnancy, has implications for preventing various types of substance use during pregnancy and for treating children affected by prenatal substance use.

Women who smoke during pregnancy are also likely to drink alcohol. In one survey, conducted as part of the Maternal Health Practices and Child Development project in Pittsburgh, Pennsylvania, 76 percent of adult women who reported smoking during their first trimester of pregnancy said that they also drank alcohol during that period. Among pregnant teenagers surveyed, 61 percent of those who smoked during the first trimester also drank alcohol. In addition, tobacco and alcohol use are both prevalent among women who use illicit drugs during pregnancy. In the National Pregnancy and Health Survey, 74 percent of women who used illicit drugs during pregnancy also reported either smoking, drinking, or both. The use of either one of these drugs is, in itself, a risk factor for poorer pregnancy outcome.

Although alcohol and tobacco are frequently used together during pregnancy, researchers studying the negative effects of prenatal exposure to tobacco and alcohol have generally examined the effects of each drug separately. Therefore, it is difficult to discuss the effects of the combined use of the two drugs. Although the other articles in this issue examine the use of alcohol and tobacco together, this article focuses on tobacco use during pregnancy and the effects of prenatal tobacco exposure. Understanding the effects of prenatal tobacco exposure allows the identification of those characteristics that are uniquely related to tobacco and those that are affected by alcohol exposure. This research, along with research on the effects of alcohol use during pregnancy, has implications for preventing various types of substance use during pregnancy and for treating children affected by prenatal substance use.

Prenatal tobacco exposure has been reported to be a significant risk factor for sudden infant death syndrome and is estimated to be responsible for up to 4,800 infant deaths as well as 61,000 low-birth-weight infants and 26,000 infants requiring neonatal intensive care annually. In a national survey of pregnant adult women, however, 20.4 percent reported smoking cigarettes during pregnancy. This proportion rises to about one-half for women in lower socioeconomic populations.

Smoking during pregnancy is more prevalent among Caucasian women compared with African-American or Hispanic women. Caucasian women also smoke at higher levels than do women of other ethnicities.Women who smoke during pregnancy are less likely to be married, have less education, have lower incomes, and attend fewer prenatal visits compared with women who do not smoke during pregnancy.

Compared with alcohol, marijuana, and other illicit drug use, tobacco use is less likely to decline as the pregnancy progresses. In the National Pregnancy and Health Study, approximately two-thirds of the women who smoked prior to their pregnancy continued smoking into the last trimester. In contrast, only one-fourth of the women who used alcohol prior to conception continued to drink into the third trimester. Women who smoke during pregnancy also continue smoking after the pregnancy. Therefore, children born to women who use tobacco during pregnancy are likely to continue to be exposed to tobacco after birth. This environmental, or passive, exposure may also affect the children's development.

Effects of Smoking During Pregnancy

This section of the article reviews findings on the effects of maternal smoking during pregnancy on the exposed children's growth, cognitive function, and behavior. The subsequent section focuses on the effects of passive smoking. Because of space limitations, this article does not review research on the effects of prenatal alcohol exposure. Because women who smoke during pregnancy are also likely to drink alcohol and use other drugs, many of the studies reviewed here controlled for prenatal alcohol exposure and other confounding factors, to determine the unique effects associated with prenatal tobacco exposure.

Effects on Infant Growth

Maternal smoking during pregnancy has long been considered an important risk factor for LBW. This association was first reported in 1957 and has been proven in numerous subsequent studies. Birth weight decreases in direct proportion to the number of cigarettes smoked, and children of smokers are 150 to 250 grams lighter than are the children of nonsmokers. The reduction in infant weight is not attributable to earlier gestation, because infants of smokers exhibit growth retardation at all gestational ages. In a recent study of neonatal body composition, prenatal tobacco exposure was significantly related to having less fat-free mass, as measured by total body electrical conductivity. The authors concluded that the LBW of infants exposed to prenatal smoking is primarily attributable to reduced fatfree mass or lean tissue. Birth length and head and chest circumference are also reduced in infants who are prenatally exposed to tobacco.

In a recent study of pregnant teenagers, more than one-half of whom were smokers, prenatal tobacco exposure was significantly related to reduced birth weight, birth length, head circumference, and chest circumference. These reductions were even more pronounced than those found in a similar cohort of the children of adult women. For example, in the study of adult mothers and their children, prenatal tobacco use was significantly associated with a reduction in birth weight of 158 grams per pack per day. In the children of teenage mothers, prenatal tobacco exposure was significantly associated with a reduction in birth weight of 202 grams per pack per day. The increased problems associated with young maternal age and poor fetal outcomes, coupled with the high prevalence of smoking among pregnant teenagers, magnify the risks to children of pregnant teenagers who smoke.

In another recent study on prenatal tobacco exposure and fetal growth, Zaren and colleagues reported that the male fetus might be more adversely affected than the female fetus. In this study, fetuses of nonsmoking, light smoking, and heavy smoking mothers were measured by sonograms at weeks 17, 25, 33, and 37. Boys born to heavy-smoking mothers had greater weight reductions, lower fat accretions, and smaller head circumferences when compared with girls of heavy smoking mothers.

Two key ingredients of cigarette smoke that are known to affect fetal growth are carbon monoxide and nicotine. Carbon monoxide causes fetal hypoxia, a reduction in the amount of oxygen available to the fetus, whereas nicotine can lead to a decrease in the flow of oxygen and other nutrients across the placenta by constricting uterine arteries. In addition, nicotine itself can cross the placenta to affect the fetal cardiovascular and central nervous systems. Other constituents of tobacco smoke (cadmium and toluene) have also been shown to cause fetal growth retardation.

About the Author

NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.