Smoking During and After Pregnancy

Effects on Growth and Cognitive Function
by National Institute of Health

(Page 2 of 4)

Long-Term Effects on Growth

The effects of prenatal tobacco exposure on older children's growth are not as clear as the effects on infants. Using data from the Collaborative Perinatal Project, Naeye detected a small difference in height and head circumference in exposed children at age 7. Rantakallio found that exposed children were shorter than nonexposed children at age 14, and Fogelman and Manor reported decreased height at ages 7, 11, and 23. In the latter study, the differences in height at age 23 were mediated by birth weight. These studies did not control for passive exposure to tobacco smoke or exposure to alcohol. A study of 714 three-year-old children found that the children of women who quit smoking during pregnancy were heavier and taller than those of women who did not quit. Adjustment for postpartum exposure to tobacco smoke reduced the difference in the children's weight, but had little effect on differences in height.

Other studies have not found growth retardation over the long-term. In addition, one study that followed infants from birth through 6.5 months and 13 months found that prenatal alcohol exposure, rather than tobacco exposure, was associated with a slower growth rate when the exposed children were compared with unexposed children during the first 6.5 postpartum months. Although maternal smoking was correlated with shorter stature at 6.5 and 13 months, this effect was attributable to maternal drinking during pregnancy, highlighting the importance of controlling for the effects of other drugs.

The MHPCD study of adult mothers and their children, which controlled for prenatal alcohol and other drug exposures and current maternal tobacco use, found a significant inverse relationship between maternal tobacco use during pregnancy and the infant's weight, length, and head circumference at birth. At 8 months, only the infant's length continued to be associated with prenatal tobacco exposure. When the children were followed up at 18 months and 6 years, prenatal tobacco exposure was not related to any growth reductions after controlling for the appropriate covariates. Similarly, Vik and colleagues found that the reductions in birth weight that were attributed to prenatal tobacco exposure were not evident when the children were 5 years old.

Prenatal tobacco exposure may not only be related to size deficits at birth, but may also be associated with disproportionate weight among both infants and young children. For example, a recent study of more than 200,000 births in Sweden found that prenatal tobacco exposure was significantly associated with reduced birth length and birth weight. However, maternal smoking was also significantly associated with an increase in ponderal index, an indication of higher proportionate weight for height, when birth weight and gestational age were controlled for. Thus, the children of smokers tended to be shorter and have a higher ponderal index than children of nonsmokers. This finding is consistent with studies that have followed children after infancy. For example, Fried and colleagues found that prenatal tobacco exposure was related to an increased rate of obesity among 6-year-olds. The researchers proposed that this association was attributable to a preference for bottlefeeding among mothers who smoked during pregnancy. Vik and colleagues also reported a higher ponderal index and increased skinfold thickness (a measure of percentage body fat) in children whose mothers smoked during pregnancy, compared with children whose mothers did not smoke. Researchers evaluating the MHPCD cohort of 6-year-old children of teenage mothers also found a positive association between prenatal tobacco exposure and increased skinfold thickness. Prenatally exposed children also had higher values on the body mass index and weightfor- height Z-scores, an indication that the children were overweight for their height. Bottle-feeding was not a significant factor. Thus, several recent studies indicate that prenatal tobacco exposure seems to alter the relationship between body length and weight. This finding is underscored by two studies that have found that prenatal tobacco exposure reduces the growth of the long bones in the fetus.

Effects on Cognitive Function

Laboratory research with animals has shown that nicotine affects the CNS at exposure levels below those at which growth changes are evident. For example, animal studies have shown associations between fetal nicotine exposure and increased locomotor activity in male rat pups; hyperactivity in rats associated with increased nicotine receptors in the brain; lower turnover of the brain chemicals dopamine and serotonin in the rat brain as a result of alterations in the release or removal of dopamine and serotonin from the synapse; and changes in the morphology of the hippocampus in rats.

In the literature on humans, prenatal tobacco exposure has also been linked to CNS effects, including cognitive and neurobehavioral outcomes, although the reports are inconsistent. At birth, prenatal tobacco exposure has been associated with poorer auditory orientation and autonomic regulation and increased tremors and startles. In a recent race-matched study of cocaineexposed and non-cocaine-exposed infants, neurological exams showed that prenatal tobacco exposure was significantly related to muscle tone abnormalities when controlling for other variables, including prenatal cocaine and ethanol exposure, head circumference, and prenatal care. The authors concluded that maternal cigarette smoking, rather than cocaine exposure, might be the major predictor of tone abnormalities.

Studies have also reported adverse effects of prenatal tobacco exposure on cognitive and behavioral development in older children. In one study, cognitive functioning at age 3 was higher among the children of mothers who quit smoking during pregnancy than among children whose mothers smoked throughout pregnancy. Poor language development and lower cognitive scores have also been reported in 2-, 3-, and 4-year-old children prenatally exposed to tobacco. When those children were 9 to 12 years old, prenatal tobacco exposure was negatively associated with language and reading abilities. In another analysis of this same cohort of 9 to 12 year-olds, prenatal tobacco exposure had a negative, dose-dependent association with visual perception after consideration of other potential prenatal risk factors and of pre- and post-natal secondhand smoke exposures.

Other researchers have argued that initially significant associations between prenatal tobacco exposure and cognitive development were explained better by differences in social class and the home environment. For example, after controlling for socioeconomic and environmental differences, Eskenazi and Trupin failed to find consistent relationships between prenatal tobacco exposure and performance on the Raven Colored Matrices Test, a measure of nonverbal reasoning, or the Peabody Picture Vocabulary Test. However, in the MHPCD study of adult mothers, prenatal tobacco exposure predicted deficits in visual memory and verbal learning scores on the Wide Range Assessment of Memory and Learning test and these associations remained after consideration of other factors, including socioeconomic status, maternal psychological status, home environment, other prenatal substance exposures, and current maternal tobacco and other substance use.

About the Author

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