An association between prepregnancy body mass index (BMI) and severe maternal morbidity (SMM) has been reported, but evidence has been mixed and potential explanations have not been examined. To evaluate the association between prepregnancy BMI and SMM in a large, diverse birth cohort and assess potential mediation by obesity-related co-morbidities and caesarean birth. This cohort study used linked birth certificate and hospitalisation discharge records from Californian births during 2007-2012. We assessed associations between prepregnancy BMI and SMM, and used inverse probability weighting for multiple mediators to estimate relative and absolute natural direct and indirect effects accounting for mediation by co-morbidities (hypertensive conditions, diabetes, asthma) and caesarean birth. Among 2 650 182 births, the prevalence of SMM was 1.42%. Adjusted risk ratios for the total association between prepregnancy BMI category and SMM were 1.12 (95% confidence interval [CI] 1.07, 1.18) for underweight, 1.02 (95% CI 0.99, 1.04) for overweight, 1.04 (95% CI 1.00, 1.07) for obesity class 1, 1.14 (95% CI 1.09, 1.20) for obesity class 2, and 1.28 (95% CI 1.22, 1.36) for obesity class 3 compared to women with normal weight. After accounting for mediation by co-morbidity and caesarean birth, the risk ratios were 1.19 (95% CI 1.14, 1.26) for underweight, 0.91 (95% CI 0.89, 0.94) for overweight, 0.86 (95% CI 0.84, 0.89) for obesity class 1, 0.88 (95% CI 0.84, 0.92) for obesity class 2, and 0.89 (95% CI 0.83, 0.95) for obesity class 3. Co-morbidities and caesarean birth explained an association between high prepregnancy BMI and SMM. These findings suggest that promotion of healthy prepregnancy weight, along with management of co-morbidities and support of vaginal birth in pregnant women with high BMI, could reduce the risk of SMM. However, these mediators did not reduce the elevated risk of SMM observed in women with low BMI. © 2019 John Wiley & Sons Ltd.