Studies have linked prenatal smoking to a higher risk of childhood obesity, but new findings suggest that effect may vary based on race and ethnicity.

In a study of more than 155,000 preschool children, U.S. researchers found that the link between mothers' smoking during pregnancy and their children's risk of obesity was most clear among white families.

In contrast, only heavy prenatal smoking was tied to childhood obesity among African Americans. And there was no clear evidence that it raised the odds of obesity among children of Hispanic, Native American or Asian descent.

Dr. Andrea J. Sharma and colleagues at the U.S. Centers for Disease Control and Prevention report the findings in the American Journal of Epidemiology.

Past studies have suggested that women who smoke during pregnancy may raise the odds of their child becoming obese by adolescence.

The reasons are not completely clear, but one theory has to do with the fact that prenatal smoking often leads to low birthweight. Infants born at a low weight tend to go through a rapid period of "catch-up" growth, and this accelerated growth may make them more vulnerable to excessive weight gain later in life.

Another possibility is that infants exposed to nicotine in the womb go through a form of withdrawal, and — similar to smokers trying to kick the habit — develop problems with appetite control.

Exactly why such effects would vary by race is unknown, according to Sharma's team. It's possible, they note, that their findings do not reflect an actual racial discrepancy in risk, but in mothers' reports; women of certain ethnic groups may, for example, have been more reluctant to admit to smoking during pregnancy.

However, past studies have found genetic variations in how smokers of different races metabolize nicotine — which could theoretically alter the toxic effects on the fetus. More studies are needed to explore these questions, Sharma and her colleagues say.

Regardless, they stress, there are plenty of reasons for women to quit smoking before they ever become pregnant, for both their own health and that of their children.