Physical activity during early pregnancy and infant birth size

Objective: To examine the relationship between maternal physical activity during the first half of pregnancy and infant birth size. Methods: Selfreported physical activity data collected between 2008 and 2010 and newborn anthropometrics from offspring birth records were obtained from 308 mothers in Minneapolis, Minnesota. Physical activity was categorized into light, moderate, or vigorous intensity based on metabolic equivalent (MET) score. Continuous MET-hours per week were calculated overall and by activity category. Z-scores standardized for gestational age and sex were calculated for birthweight, length, and head circumference using national birth data charts. We used linear regression to estimate associations between maternal physical activity and standardized newborn birth size, adjusting for potential confounders. Results: Total, light, and moderate physical activity were not associated with any of the birth outcomes. However, vigorous physical activity was positively associated with birthweight, length, and head circumference (all P <0.05), but not ponderal index (P = 0.8). An increase of three MET-hours per week of vigorous activity was estimated to result in a ~50 gram increase in birthweight, a ~1/3 centimeter increase in length, and a ~1/6 centimeter increase in head circumference. Conclusion: Women who engaged in greater amounts of vigorous physical activity during the first half of pregnancy tended to give birth to larger infants. Future studies are warranted to confirm these results and explore potential biological mechanisms of enhanced fetal growth associated with vigorous physical activity.