Exercise During Pregnancy, Maternal Prepregnancy Body Mass Index, and Birth Weight

OBJECTIVE: To estimate the direct associations between exercise during pregnancy and offspring birth weight and between maternal prepregnancy body mass index (BMI) and birth weight. Furthermore, we estimated the indirect association between maternal BMI and birth weight, explained by exercise during pregnancy. METHODS: This study included pregnant women and their offspring recruited from 1999 to 2006 in the Norwegian Mother and Child Cohort Study, conducted by the Norwegian Institute of Public Health. Linear regression analyses were based on exposure data from two self-administered questionnaires during pregnancy and birth weight data from the Medical Birth Registry of Norway. RESULTS: The study included 43,705 pregnancies. The median exercise frequency during the first 17 weeks of gestation was six times per month and four times per month thereafter until week 30. Mean maternal prepregnancy BMI was 24 kg/m(2), and mean birth weight of the offspring was 3,677 g. The adjusted direct association between exercise and birth weight was a 2.9-g decrease in birth weight per unit increase in exercise (one time per month). In contrast, the adjusted direct association between BMI and birth weight was a 20.3-g increase in birth weight for a one-unit increase in BMI (1 kg/m(2)), and the indirect association explained by exercise was only a 0.3-g increase in birth weight. CONCLUSION: Exercise during pregnancy has a minor impact on birth weight, whereas maternal prepregnancy BMI has a larger influence. Thus, we suggest that health care professionals should focus on normalizing the BMI of women in fertile ages. (Obstet Gynecol 2010,115:331-7)