In the US, about 1 in 8 women smokes during pregnancy. Both carbon monoxide and nicotine—agents released through smoking—may lower the oxygen available to the fetus. Other components of cigarettes have been linked to birth defects in laboratory animals as well. Although the health effects of smoking during pregnancy are well documented, until recently, evidence about smoking’s impact on birth defects was not clearcut.
SMOKING RAISES RISK FOR ORAL CLEFTS
Past studies hinted that smokers’ babies may be more likely to have cleft lip and/or cleft palate, but results were mixed. Are some infants genetically more susceptible to mothers’ smoking? The Program looked at a gene normally involved in development of the palate and mouth—the transforming growth factor-alpha gene (TGF).
|Women who smoke during pregnancy were 1.5 to 2 times as likely to have babies with oral clefts. The more cigarettes the mother smoked, the higher the risk.|
|The hazards of smoking are even greater for the 1 in 7 babies who carry a cleft-susceptibility gene (the A2 form of TGF). They were 8 times as likely to have oral clefts if their mothers smoked. Those born to nonsmoking mothers were at no greater risk.|
|Nonsmoking mothers exposed to secondhand smoke had only a small, if any, increased risk. Father’s smoking increased the risk for oral clefts only if the mother smoked too.|
|Cutting out smoking could prevent more than 200 oral clefts in California each year.|
SMOKING AND OTHER BIRTH DEFECTS
|Heart and limb defects. In other birth defects studied, the connection with smoking is not straightforward. For example, we saw a modest risk increase for conotruncal heart defects and limb defects, but only if both parents smoked. Perhaps smoking patterns are different (for instance, more cigarettes/day) when both parents smoked, or the risk increase could be due to other behaviors more common among smokers.|
|Neural tube defects. Parents’ smoking did not increase risk.|
|Down syndrome. Environmental factors—interacting with the developmental instability caused by an extra chromosome—may influence which babies have associated abnormalities. Babies with Down syndrome whose mothers smoked during the first trimester had double the risk for heart defects.|