Physical activity during pregnancy and gestational weight gain among nulliparous women: Secondary analysis of the Nulliparous Pregnancy Outcomes Study: Monitoring Mothers‐To‐Be

Objective This study examines the longitudinal association between leisure‐time physical activity during pregnancy and gestational weight gain (GWG) among nulliparous women. Methods We analyzed data from 8146 nulliparous women enrolled in the Nulliparous Pregnancy Outcomes Study: Monitoring Mothers‐To‐Be (nuMoM2b). Physical activity was assessed at three visits using questionnaires and categorized by weekly metabolic equivalent (MET)‐hours and adherence to guidelines. GWG was evaluated as the rate of weight gain during successive gestational periods and as gain at or above the 75th percentile. Multivariable regression models adjusted for demographic, lifestyle, dietary, and pregnancy‐related factors were used to examine overall and time‐dependent associations. Results Women with consistently higher physical activity levels experienced a faster rate of GWG during early pregnancy but a slower rate during later pregnancy. Compared with women reporting <7.5 MET hours/week, those reporting ≥21 MET hours/week had a greater early pregnancy rate of GWG (adjusted mean difference 0.032 kg/week, 95% confidence interval [CI] 0.009 to 0.056) but a slower later pregnancy rate (−0.026 kg/week, −0.053 to 0.000). Women who consistently met physical activity guidelines were less likely to have GWG at or above the 75th percentile before delivery (adjusted risk ratio 0.87, 95% CI 0.78 to 0.98). Time‐updated analyses further showed that higher physical activity during mid‐pregnancy was associated with a slower subsequent rate of GWG. Conclusion Leisure‐time physical activity was associated with GWG in a stage‐specific manner. These findings highlight the importance of considering gestational timing when evaluating the effects of physical activity on maternal weight gain and support continued promotion of physical activity throughout pregnancy.