NEW YORK (Reuters Health) - Exercise during pregnancy, while healthy for both mother and baby, has only a minor impact on an infant's birth weight, suggest findings from a large study from Norway.

On the other hand, the findings confirm a strong association between being heavy prior to becoming pregnant and having a heavier baby, Caroline Fleten of the Norwegian Institute of Public Health in Oslo, and colleagues report.

Fleten and colleagues evaluated associations between exercise during pregnancy, prepregnancy weight and the baby's weight at birth for 43,705 women aged 15 to 49 years old and pregnant with a single fetus.

They determined body mass index (BMI), a standard measure of how fat or thin a person is. On average, prepregnancy BMI of the women in the study was 24. For reference, a BMI of 25 or higher is considered overweight, while a BMI of 30 or higher is considered obese.

Fleten's team found that each single unit increase in mothers' prepregnancy BMI was associated with about 20 grams (0.70 ounces) heavier weight at birth. As an example, an increase in BMI of 5 units -- 29 versus 24 for example -- would result in a birth weight increase of 103 grams (3.63 ounces).

The women reported brisk walking, jogging, bicycling, aerobics, fitness or weight training, or other physical activities an average of 6 times a month during the first 17 weeks of pregnancy and then 4 times a month until week 30.

The average weight of infants at birth was 3,677 grams (about 8 pounds) and, according to the investigators, exercise during pregnancy had no significant effect on birth weight.

"Exercise during pregnancy cannot be used as a means of 'normalizing' birth weight," Fleten commented in an email to Reuters Health.

She and her colleagues suggest that health care professionals focus on preventing or treating overweight and obesity in women of childbearing age, which would reduce the risk of giving birth to a baby weighing too much.

SOURCE: Obstetrics and Gynecology, February 2010