Dr Sadia Malik

This is the first research to show specifically that septal defects are linked with maternal smoking, the lead researcher, Dr Sadia Malik (University of Arkansas for Medical Sciences, Little Rock), told heartwire. It builds on a previous study—the Baltimore/Washington Infant Study—that showed that smoking was associated with an increased risk of right-sided heart problems, she added.

"This is really another reason not to smoke," she said, "there are 35 000 infants born every year in the US with congenital heart disease, and if all women of reproductive age stopped smoking, it would result in 2000 fewer babies born with heart disease."

Malik and colleagues conducted a population-based, case-control study on 566 infants with congenital heart disease, 491 without, and their parents. Mothers of both sets of infants were asked whether they smoked from one month before pregnancy through the end of pregnancy. Women's exposure to tobacco smoke at home or work during the same period was also determined.

Of women who had children with congenital heart disease, 34% reported they smoked sometime from the month prior to conception through the end of the first trimester, compared with 25% of women whose children did not have heart problems. Malik said these figures—in an Arkansas population—were higher than the national averages in the US; nationally, 20% of women smoke in the month before pregnancy and 12% continue to smoke to the end of the first trimester.

The women who smoked were more likely than those who didn't to have infants with congenital heart disease (odds ratio 1.6), independent of confounding factors. And in subgroup analysis, those who smoked were 80% more likely to have an infant with a septal or right-sided obstructed heart defect than women who had a baby without a heart defect.

"The heart's basic structures develop very early in pregnancy, before many women realize they are pregnant," Malik noted. "Thus, even if a woman quits smoking at six weeks or later, her fetus will still have been exposed to the harmful effects of cigarette smoking during cardiac development."

"Any woman of reproductive age, whether planning pregnancy or not—because a lot of pregnancies are unplanned—should not smoke," she told heartwire.

Malik says it is important that healthcare providers who talk directly to mothers "keep this in their consciousness. I hope this jogs their memory, so that they think, 'You know what, there's another study out.' It makes a big difference."

Surprisingly, however, the new research did not reveal a link between passive smoking and congenital heart defects, but she says this may simply be because their sample size was too small.

Malik is part of the National Birth Defects Study, which she says is "the largest case-control study in the US," tracking 5000 cases of congenital heart disease and an equal or greater number of controls in 10 states. Among the things they are examining is whether some women might have a genetic predisposition that makes them particularly susceptible to the effects of cigarette smoke.