Obstetrics and Gynecology, Feb 2010 published a study out of Norway looking at exercise, BMI, and baby birth weight.

Findings?

They looked at 43000+ women aged 15-49 who were pregnant with a single fetus.  The women’s exercise was walking jogging, biking, weight training, aerobics, etc.  They averaged 6 times a month for the first half of pregnancy, and then once a week until week 30. 

The average weight of the infants at birth was 3,677 grams (8 pounds. Ouch!), and those who exercised during pregnancy did not have a significant effect on birth weight.

BUT, they did find an association with BMI. 

What is BMI? BMI is body mass index, to do it in US measurements, it is 703 x weight (lb)/ height (inches) squared.  BMI 18.5-23.9 is normal, 24-29.9 is overweight, and greater than 30 is obese.

The prepregnancy average BMI of the women in the study was 24. Fleten’s team found each unit increase in the mother’s BMI was associated with 20 grams (0.70 ounces) heavier birth weight.  So an increase in BMI of 5 units — 29 versus 24 — would cause a birth weight increase of 103 grams (3.63 ounces).

The Norwegian doctors suggest doctors focus on preventing or treating overweight and obese women of childbearing age to help reduce the risk of giving birth to babies who weigh too much. (OUCH!)

SOURCE: Obstetrics and Gynecology, February 2010