Women who smoke during pregnancy may be raising their child’s risk of becoming obese by adolescence, an Australian study suggests.

Researchers found that 14-year-olds whose mothers smoked during pregnancy were more likely to be overweight or obese than children of non-smoking women. The link was independent of factors like family income, mothers’ education and the children’s diet and exercise habits.

The findings suggest that a mother’s prenatal smoking has a “direct effect” on her child’s weight later on, the study authors report in the American Journal of Epidemiology.

The researchers, led by Dr. Abdullah Al Mamun of the University of Queensland, based their findings on data from 3,253 women and their children.

The mothers had been followed since pregnancy, when their smoking habits were assessed; the children had their weight and height measured at age 14, at which time the mothers reported on their children’s diet and physical activity levels.

In all, the study found, more than one-third of mothers smoked during pregnancy. Their children were less likely to have been breastfed, tended to have a poorer diet later in childhood and watched more TV than children whose mothers didn’t smoke during pregnancy.

But even with these factors considered, teens who were exposed to prenatal smoking were about one-third more likely to be overweight or obese than their peers were.

Given the hundreds of chemicals in tobacco smoke, it’s difficult to pinpoint why prenatal exposure might have lasting effects on children’s weight, according to Al Mamun and his colleagues.

Maternal smoking during pregnancy can cause low birthweight, which in turn often spurs a rapid “catch-up” growth in infancy. This growth could have lingering metabolic effects that make children prone to excess pounds.

Researchers also speculate that nicotine can affect the fetal brain in a way that influences appetite control throughout life.

Whatever the reason, Al Mamun’s team concludes, these latest findings “provide yet another incentive for pregnant women to be persuaded not to smoke and for young women to be encouraged never to take up smoking.”

SOURCE: American Journal of Epidemiology, August 15, 2006.