Association of Pre-pregnancy BMI and Postpartum Weight Retention Before Second Pregnancy, Washington State, 2003-2013

Background Maternal overweight and obesity is one of the most common high-risk obstetric conditions associated with adverse birth outcomes. Smaller studies have suggested that pre-pregnancy body mass index (BMI) is associated with postpartum weight retention. Objective The primary objective of this study was to examine the association between pre-pregnancy BMI status and maternal weight retention. Study design We conducted a population-based retrospective cohort study using Washington State birth certificate data from 2003-2013. We included women who had two sequential births during this time period, with the second birth occurring within 18-36 months of the first singleton delivery date. BMI before a women's first pregnancy ("pre-pregnancy BMI") was categorized as normal (18.5-24.9 kg/m²) and overweight/obese (25-40 kg/m²). Women were classified as having returned to first pre-pregnancy BMI if their BMI before their second pregnancy was no more than 1 kg/m² more compared to their BMI before their first pregnancy. Analyses were stratified by gestational weight gain during the first pregnancy (below, met, exceeded recommended gestational weight gain). Results A total of 49,132 mothers were included in the study. Among women who met their recommended gestational weight gain, compared to mothers with a normal BMI, obese/overweight mothers were less likely to return to their pre-pregnancy BMI (76.5 vs 72.3%; RR_{Obese/Overweight} = 0.88; 95% CI: 0.85-0.92). A similar pattern was observed among women who exceeded their recommended gestational weight gain (62.6 vs 53.2%; RR_{Obese/Overweight} = 0.79, 95% CI: 0.78-0.80). Conclusion Pre-pregnancy BMI in the overweight/obese range is associated with a decreased likelihood of returning to pre-pregnancy BMI. Further research to support women during and after their pregnancy to promote behavior changes that prevent excessive weight gain during pregnancy and weight retention after birth is needed.