BMI affects macrosomia more than exercise during pregnancy

A woman's prepregnancy body mass index (BMI) appears to wield greater influence over her baby's birth weight than the amount she exercises during pregnancy, according to the findings of a Norwegian cohort study.

Conducted by the Norwegian Institute of Public Health, the study included 43,705 pregnancies.

Researchers calculated that the association between maternal exercise during pregnancy and fetal birth weight, after adjusting for pregnancy-related issues, socioeconomic status, environmental exposures, and health and lifestyle factors and including maternal prepregnancy BMI (mean, 24 kg/m² ) in the equation, was a 2.1-g decrease in birth weight per unit increase in exercise (1 time per month) around week 17 of pregnancy and a 1.4-g decrease in birth weight around week 30. Thus, a baby born to a woman who exercises 20 times per month or 5 times per week during pregnancy would weigh 42 g less than a baby born to a sedentary woman.

Fleten C, Stigum H, Magnus P, Nystad W. Exercise during pregnancy, maternal prepregnancy body mass index, and birth weight. Obstet Gynecol. 2010;115(2 pt 1):331-337.