Women who work out while they’re pregnant have slimmer babies, new research from New Zealand shows.

Because babies born to moms who didn’t exercise had higher-than-average weights, the study’s authors say, exercise could have helped “normalize” the weights of the exercisers’ babies.

“The modest reduction in birth weight in this study may lead to a long-term reduction in the risk for obesity in offspring of women who exercised in pregnancy,” Dr. Paul L. Hofman of the University of Auckland and his colleagues write.

The team randomly assigned 84 women pregnant with their first baby to up to five 40-minute sessions on an exercise bike at home each week or to a no-exercise group, beginning at 20 weeks of pregnancy through delivery.

While there were no differences in the women’s body mass index (a measure of weight in relation to height used to gauge whether someone is overweight or obese) or weight, the women who exercised were fitter than those who didn’t.

The babies born to the exercisers were about 140 grams (5 ounces) lighter than the control groups, although their average length was the same, leading to a lower BMI. This “modest shift” in weight was still within the normal range, and none of these babies were abnormally small for their age at birth.

Hofman and his colleagues will continue to follow the children to observe their long-term growth patterns.

The researchers had hypothesized that exercise might help maintain the women’s sensitivity to the blood-sugar-regulating hormone insulin, which tends to decline in pregnancy. But there was no difference in insulin sensitivity between the exercisers and the non-exercisers.

The lack of an effect “may be a reassuring finding,” Hofman and his team say, given that excessive insulin sensitivity in late pregnancy may limit fetal growth.

SOURCE: Journal of Clinical Endocrinology & Metabolism, May 2010.