Prepregnancy body mass index, pregnancy weight gain, and postpartum weight retention in Pennsylvania 2003 to 2020: an age-period-cohort analysis

The population-level increase in prepregnancy obesity affects pregnancy weight gain and postpartum weight retention, contributing to an intergenerational cycle of obesity. This study evaluated age, period, and cohort effects in prepregnancy BMI, excessive pregnancy weight gain, and postpartum weight retention among 811,111 primiparous Pennsylvania births from 2003 to 2020. We used hierarchical age-period-cohort cross-classified random effects models to estimate age-period-cohort effects in prepregnancy BMI, as well as excessive weight gain and postpartum weight retention stratified by BMI category. The 1.8-kg/m2 acceleration of prepregnancy BMI from 2003 to 2020 occurred to a similar degree in all age groups and birth cohorts. There were no main effects of delivery period or birth cohort on excessive weight gain beyond age-related variation in any BMI group. Over the same period, weight retention between the first and second pregnancies (n=382,328) increased faster with each younger generation of women who were normal weight in their first pregnancies, but to a comparable extent in all generations of women in all other BMI groups. Our data suggest that the acceleration in prepregnancy BMI and postpartum weight retention will continue unless structural and institutional changes are made to systems and policies necessary to promote healthy nutrition for all women.