New York: Taller women who participate in regular physical exercise during pregnancy may deliver lighter babies, but still within the "normal" birth weight range, which could have health benefits for the baby and the mother, a study shows.

"Our findings support that healthy pregnant women, with no obstetric complications, can perform appropriate physical activity during pregnancy and does not adversely affect birth weight," study chief Dr. Cooker C. D. Perkins from Pepperdine University, Malibu, California, told Reuters Health. "In fact, physical activity may positively affect birth weight, and subsequently positively affect labor, in taller women," she added.

In 51 healthy, non smoking women, Perkins and colleagues examined the potential impact of physical activity during pregnancy on infant birth weight and compared that to well-established predictors of birth weight, such as weight gain during pregnancy and maternal height. As expected, they found a relationship between physical activity during pregnancy and lower birth weight.

Women who were active tended to have smaller babies compared with women who were not active. On closer look, the relationship between physical activity during pregnancy and birth weight was only truly evident in the women who were taller than the average height of the group --those who were taller than about 5 feet 5 inches.

"The obvious answer would be that the taller women were more active, but it was not the case in our subjects," said Perkins. One possible explanation may be that in shorter women, there is less room for the baby to grow in excess. The shorter women, whether physically active or sedentary, had babies about the same size, and the physically active, taller women also had babies about the same size.

However, taller, sedentary women had larger babies than the other three groups (taller active, shorter active, and shorter sedentary).

"This could mean that being sedentary is a risk factor for excess fetal growth in tall women whose frames may allow for the additional fetal growth," Perkins said.

Because higher infant birth weight "has been related to labor complications,perhaps tall and sedentary is a risk for labor complications."

Perkins emphasized that all of the women delivered healthy, normal-weight babies, regardless of their physical activity level.

"However, differences in birth weight, even when within the range of 'normal' may predispose a child for certain health outcomes."

For example, the difference in birth weights of the women with the lowest and the highest level of physical activity - about 600 grams -- has been associated with blood pressure differences when the child reaches adulthood, Perkins and colleagues note.

SOURCE: Obstetrics and Gynecology, January 2007.