Discoveries and Data: Exposures and Risk Factors

discoveries and data center

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cleft lip
cleft palate
conotruncal heart defects
Down syndrome
limb defects
neural tube defects

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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.

1 IN 4 CALIFORNIA WOMEN SMOKE

	74% of mothers who had normal infants did not smoke in the 3 months before conception. 17% smoked less than a pack/day; 9.5% reported smoking 1-2 packs/day.

	Smoking was rarer in foreign-born Latinas—87% did not smoke.

	Only 23% of smokers quit in the first trimester.

	Infants who have a variant of NAT1, an enzyme that normally helps process chemicals from cigarette smoke, are more likely to have an oral cleft if their mothers smoked during pregnancy.

	Women who smoke increase the risk for clefts among babies who have a gene variation of Glutathion S-Transferases, another enzyme that helps process chemicals from cigarette smoke.