Cleft lip and cleft palate are among the most common types of birth defect. They arise when the tissues that form the roof of the mouth and the upper lip do not fuse properly, sometime between the fifth and ninth week of pregnancy.
In the current study, Norwegian researchers found that women who smoked more than 10 cigarettes per day during their first trimester were nearly twice as likely to have a baby with a cleft lip as nonsmokers were.
Similarly, nonsmoking women who were near a smoker for at least two hours each day had a 60 percent higher risk than women who were not exposed to passive smoking.
Researchers led by Dr. Rolv T. Lie, of the University of Bergen in Norway, report the results in the journal Epidemiology.
Past studies have linked mothers' smoking to cleft lip and, less consistently, to cleft palate.
The new findings add that to that evidence, and also suggest that smoking affects the odds of cleft lip regardless of certain genes.
Lie's team assessed 1 336 infants - 573 of whom had an oral cleft - for several variations in "detoxification" genes believed to help the body rid itself of tobacco smoke toxins. In most cases, their parents were assessed as well.
In theory, certain variations in these genes may make people more or less vulnerable to the toxic effects of tobacco smoke. However, Lie's team found no evidence that these genes affected the cleft lip risk connected with maternal smoking and passive smoking.
"First trimester smoking was clearly associated with risk of cleft lip," the researchers conclude. "This effect was not modified by variants of genes related to detoxification of compounds of cigarette smoke."