Physical activity during pregnancy is recommended for its many benefits for both mother and fetus, including reducing the risk of pregnancy complications, perinatal depression and risk factors for childhood obesity, such as high birthweight. Many people of reproductive age are not active, and pregnancy can be a motivating time to start physical activity. Fortunately, current guidelines encourage inactive individuals to safely initiate physical activity during pregnancy.
But when? Since we can’t add time to the 24-hour day, we have to instead replace time spent in another behavior with physical activity. This brings up two other important questions, though: Does it matter which behaviors we replace? And are certain behavior replacements more beneficial than others?
The current physical activity guidelines give some basic direction: “sit less, move more,” implying that replacing sedentary time with higher intensity physical activity is beneficial. However, it has so far been unclear to what level this affects infant growth outcomes during pregnancy.
Our study, published in the February 2025 issue of Medicine & Science in Sports & Exercise®, addresses the question of how various theoretical behavior replacements to increase levels of moderate to vigorous physical activity (MVPA) during pregnancy are associated with infant growth outcomes. We used data from 116 pregnant individuals with pre-pregnancy overweight or obesity who provided information about waking movement and sleep duration in early and late pregnancy as well as birthweight, neonatal adiposity and infant growth.
We combined accelerometer data and self-reported sleep duration to create 24-hour movement profiles, reflecting the proportion of the 24-hour period spent in sleep, sedentary behavior, light intensity physical activity (LPA) and MVPA. We then applied a statistical approach (compositional data analysis) that accounts for the interrelatedness of behaviors within the 24-hour movement profile to model how theoretical behavior substitutions in early and late pregnancy impact infant growth outcomes and whether initial levels of MVPA or sleep affected these associations.
We found that reallocating 10 or more minutes of any other behavior in the 24-hour movement profile (i.e., sleep, sedentary behavior or LPA) to MVPA in either early or late pregnancy was associated with a lower risk of high birthweight. We additionally found that in early pregnancy, reallocating time from any other behavior to MVPA was associated with lower risk of rapid infant growth. These results suggest that, at least in individuals with pre-pregnancy overweight or obesity, no specific behavior replacement is more beneficial than any other for infant growth outcomes. We also observed that (a) associations were similar in those with low and adequate sleep and (b) were stronger when increasing MVPA from initially low levels. This aligns with previous studies in adults, which report that the greatest cardiovascular and metabolic benefits occur when going from no to some MVPA.
Our results reinforce current physical activity guidelines during pregnancy and contribute to the understanding of how behavior change, including increasing MVPA, within the 24-hour movement framework during pregnancy impacts infant growth. This is critical as future physical activity and 24-hour activity guidelines are developed for pregnant individuals.
Sylvia Badon, PhD, is a perinatal and women’s health epidemiologist at the Kaiser Permanente Northern California Division of Research. Her research is dedicated to improving the lives of women and children by examining how physical activity, sleep and other lifestyle behaviors contribute to physical and mental health throughout the lifecourse. Dr. Badon is an active collaborator in several large, multisite investigations and is leading an ongoing study assessing 24-hour activity profiles during pregnancy funded by a NICHD R00 Pathway to Independence award. She is a current member of the ACSM Research Review Committee and the Pregnancy and Postpartum Special Interest Group.
