Regular exercise during pregnancy doesn’t change a mother’s body weight but does lead to a small reduction in the baby’s weight, a new study shows.

The findings are important because larger birth size (babies weighing more than 8 pounds 12 ounces) is associated with higher risk for childhood obesity.

In a randomized trial of 84 first-time mothers, participants were assigned to either an exercise or a control group. Exercisers rode stationary bikes for 40 minutes, five days a week, while the other women just maintained their regular activity. The women began exercising in the 20th week of pregnancy and continued until at least week 36.

The exercise didn’t make any difference in the mother’s weight compared with mothers who weren’t exercising. But among mothers who didn’t exercise, their babies were an average of five ounces heavier than babies born to women who exercised during pregnancy. There also was no difference in the average length of the babies, according to the study published in The Journal of Clinical Endocrinology and Metabolism.

The study shows that moderate, regular aerobic exercise can have a meaningful effect on a baby’s birth weight without putting the pregnancy or child at risk.