Maternal prepregnancy body mass index and physical activity during pregnancy assessed by accelerometer

Background: Maternal prepregnancy overweight and obesity increase the risk of adverse pregnancy outcomes, whereas physical activity during pregnancy has a beneficial effect on both the mother and the fetus. Limited data are available on how maternal prepregnancy overweight and obesity affect physical activity during pregnancy.

Objective: The purpose of this study was to describe the association between prepregnancy body mass index and physical activity during pregnancy.

Study design: An observational prospective cohort study of 400 singleton pregnant women who were attending routine antenatal care at Aarhus University Hospital, Denmark (2010-2015), was conducted. Physical activity was assessed by an accelerometer (SenseWear Armband) for 7 days for each trimester. Participants were stratified in 3 different groups of prepregnancy body mass index: normal weight (body mass index <25 kg/m²), overweight (body mass index 25-29.9 kg/m²), and obese (body mass index ≥30 kg/m²). Physical activity was measured as the number of steps per day, metabolic equivalent of task per day, time in moderate- to vigorous-intensity physical activity (>3 metabolic equivalent of task), and time in vigorous-intensity physical activity (>6 metabolic equivalent of task). Linear regression and multilevel mixed-effects models were used to explore the association between prepregnancy body mass index and physical activity variables during pregnancy.

Results: We found an inverse linear relationship between prepregnancy body mass index and both mean number of steps per day and mean metabolic equivalent of task per day (P<.001). At baseline, women with normal weight walked a median of 1214 steps per day (95% confidence interval, 576-1852) more than women who were obese (P<.05), and women who were overweight walked a median of 948 steps per day (95% confidence interval, 218-1677) more than women who were obese (P<.05). Independent of prepregnancy body mass index, all variables of physical activity decreased over the course of pregnancy (P<.05), with the greatest decrease in the third trimester.

Conclusion: Maternal physical activity measured by an accelerometer decreased across pregnancy independent of maternal body mass index status and was inversely associated with prepregnancy body mass index. Thus, being overweight or obese before pregnancy increased the risk of sedentary behavior during pregnancy.