Active Taller Moms Have Smaller Babies

Jan. 5, 2007 -- Exercise during pregnancy can keep taller women from having overly large babies, a small study suggests.

Low birth weight signals health problems. But very large babies can mean difficult labor and cesarean deliveries.

Some things are known to affect a baby's birth weight. Women who smoke during pregnancy have lower-weight babies. Women who have higher than normal weight gain during pregnancy have higher-weight babies.

But for taller women -- those over 5 feet 5 inches -- exercise during pregnancy appears to have a greater effect on the child's birth weight than either smoking or excessive weight gain.

Babies born to the women most active during their pregnancies weigh on average about 1 pound, 5 ounces less than those born to the most sedentary women, report Cooker C.D. Perkins, PhD, of Pepperdine University in Malibu, Calif., and colleagues.

The researchers studied 51 healthy, nonsmoking white women during their pregnancy, collecting detailed records of the women's physical activity.

This included two 48-hour periods (at weeks 20 and 32 of pregnancy) during which the women wore devices that measured the duration and intensity of their activities.

All the women were healthy, and all their pregnancies were normal.

Children born to the women ranged in weight from about 6 pounds to about 11 pounds.

The most active women had babies at the lower end of the normal range for birth weight. But this effect was almost exclusively limited to women taller than 5-foot-5.

"Fetal growth may be sufficiently constrained by maternal size in shorter women," Perkins and colleagues suggest, "so that physical activity during pregnancy has less opportunity to impact on fetal growth.

"In taller women, with fewer constraints on fetal growth, physical activity may be more important than maternal size in setting limits on birth weight," the researchers write.

Because the study focused on healthy women who had normal birth weight babies, the scientists stress that the findings do not apply to the problem of low birth weight.

Their findings appear in the January issue of Obstetrics & Gynecology.