Theoretical Behavior Substitutions during the 24-Hour Day in Pregnancy and Infant Growth Outcomes.

Purpose: To assess associations of theoretically reallocating time from sleep, sedentary behavior, or light intensity physical activity (LPA) to moderate/vigorous intensity physical activity (MVPA) during pregnancy with infant growth outcomes.

Methods: We used data from a cohort of pregnant individuals with overweight or obesity (n = 116). At 9-15 and 30-36 weeks gestation, waking movement was measured using wrist-worn accelerometers and sleep duration was self-reported. Outcomes were obtained from delivery electronic health records (birth) and study visits (12 months). We used compositional isotemporal substitution models.

Results: In early pregnancy, reallocating 10 minutes of sleep, sedentary behavior, or LPA to MVPA was associated with 20% (RR = 0.80; 95%CI: 0.75,0.85), 21% (RR = 0.79; 95%CI: 0.75,0.84), and 25% (RR = 0.75; 95%CI: 0.70,0.81) lower risk of large-for-gestational age (LGA) birthweight, respectively, and 17% (RR = 0.83; 95%CI: 0.75,0.91), 18% (RR = 0.82; 95%CI: 0.75,0.91), and 22% (RR = 0.78; 95%CI: 0.70,0.88) lower risk of rapid infant growth (birth to 12 months), respectively. In late pregnancy, reallocating 10 minutes to MVPA was associated with 18% to 22% lower risk of LGA birthweight, but was not associated with rapid infant growth. Reallocating time to MVPA in early or late pregnancy was not associated with high newborn body fat percent.

Conclusions: Our findings suggest beneficial associations of theoretically reallocating time from sleep, sedentary behavior, or LPA to MVPA, especially during early pregnancy, for reducing LGA birthweight and rapid infant growth.