Population-based associations between maternal pre-pregnancy body mass index and spontaneous and medically indicated preterm birth using restricted cubic splines in California

The literature pertaining to risk of spontaneous preterm birth (sPTB) as related to body mass index (BMI), specifically high BMI, is conflicting.To assess the relationships between maternal prepregnancy BMI and sPTB separately for non-Hispanic Whites, non-Hispanic Blacks, Hispanics, and Asians.Population-based cohort study of mothers who delivered a singleton livebirth in California from 2007 to 2012. Associations between BMI and sPTB were estimated from Cox proportional hazard models. BMI was modelled with restricted cubic splines to account for nonlinear relationships.A total of 2,645,950 births were included in the analysis, 135,357 (5.12%) in which the mother had a sPTB. Compared to mothers within the same race/ethnicity and a BMI of 26 kg/m2, all mothers with a BMI 28 kg/m2 or higher had significantly elevated adjusted hazard ratios for sPTB. Asian mothers with a BMI between 16 and 25 kg/m2 had significantly decreased hazard ratios for sPTB while a of BMI 20 kg/m2 or less among non-Hispanic White, non-Hispanic Black, and Hispanic mothers showed increased hazard ratios.This study observed that mothers with high prepregnancy BMIs were more likely to experience sPTB across all race/ethnicities.