Leonard SA, et al. - Using linked birth certificate and hospitalization discharge records from Californian births during 2007-2012, researchers conducted this cohort study to explore the connection between prepregnancy BMI and severe maternal morbidity (SMM) in a large, diverse birth cohort and evaluate potential mediation by obesity-related co-morbidities (hypertensive conditions, diabetes, asthma) and caesarean birth. Among 2,650,182 births, SMM prevalence was 1.42%. The association between high pregnancy BMI and SMM was explained by co-morbidities and caesarean birth. Such findings suggest that promoting healthy pregnancy weight in pregnant women with high BMI, along with managing co-morbidities and endorsing vaginal birth, may reduce the risk of SMM. Such mediators, however, did not reduce the elevated SMM risk observed in low BMI females.