Association Between Change in Physical Activity During Pregnancy and Infant Birth Weight

We assessed whether total, recreational, and non-recreational physical activity (PA) assessed twice during pregnancy, and its change, were associated with infant birth weight and small for gestational age (SGA). We included 1467 Pregnancy, Infection, and Nutrition 3 Study participants who self-reported PA at time 1 (T1: 17–22 weeks’ gestation) and time 2 (T2: 27–30 weeks’ gestation). We assessed last week absolute intensities of PA (moderate: 4.7–7.1 METs; and vigorous: > 7.1 METs) and perceived intensities. Change in hours/week of PA was assessed continuously or categorically (increase or decrease ≥ 1 hour, and no change). Associations of continuous PA hours/week at T1, T2, and its change, with sex-specific z-scores of birth weight, were assessed using multivariable linear robust regressions. We used logistic regressions to assess categorical PA measures with SGA. Models were adjusted for adequacy of maternal weight gain, general health, maternal age, parity, race/ethnicity, and smoking. Hours/week of total and recreational absolute intensities of PA at T1, T2, and its change were generally not associated with birth weight, although two measures of non-recreational PA at T2 and its change were associated with increased birth weight. Perceived intensities of PA (at T1, T2, and its change) were largely not associated with sex-specific z-scores of infant birth weight. Absolute and perceived intensity PA were not associated with SGA. In this observational cohort, increases and decreases in PA during pregnancy were not associated with differential changes in birthweight or SGA. Previous evidence about how physical activity (PA) during pregnancy influences birth weight is conflicting. This may be because experimental studies often require that pregnant individuals change their PA habits, while observational studies may reflect a continuation of pre-pregnancy habits for some participants. Self-selected changes in PA during pregnancy in relation to infant birth weight outcomes are understudied. Self-selected changes in physical activity during pregnancy are largely not associated with infant birth weight outcomes in this study. Findings from this study can inform PA recommendations for pregnant individuals, and could assure them that maintaining or increasing PA during pregnancy is safe in regards to infant birth size.