Weight Gain During Pregnancy and Severe Maternal Morbidity

Importance Severe maternal morbidity (SMM) carries serious consequences for pregnant individuals, yet the association between gestational weight gain (GWG) and SMM remains poorly understood. Objective To examine the association between GWG and SMM across prepregnancy body mass index (BMI) categories. Design, Setting, and Participants This retrospective cohort study used linked birth registry data from Better Outcomes Registry & Network Ontario and hospitalization data from the Canadian Institute for Health Information for singleton pregnancies with a hospital delivery at a gestational age of 20 weeks or greater in Ontario, Canada, from April 1, 2012, to March 31, 2022. Data were analyzed from May 1 to November 1, 2025. Exposures Continuous GWG as mean weekly gain in the second and third trimesters and categorical GWG (inadequate, adequate, or excessive) according to the 2009 Institute of Medicine recommendations based on prepregnancy BMI. Main Outcomes and Measures SMM, defined as a composite of 13 indicators per the Canadian Perinatal Surveillance System, occurring during delivery hospitalization. Results Among 1 035 927 pregnancies, 192 731 (18.6%) had inadequate, 296 448 (28.6%) had adequate, and 546 748 (52.8%) had excessive GWG; 15 119 (14.59 per 1000 births) experienced SMM. Compared with adequate GWG, excessive GWG was associated with increased SMM across all BMI categories: underweight (adjusted rate ratio [ARR], 1.33 [95% CI, 1.15-1.53]), normal weight (ARR, 1.24 [95% CI, 1.17-1.31]), overweight (ARR, 1.16 [95% CI, 1.06-1.27]), obesity class I (ARR, 1.21 [95% CI, 1.08-1.35]), obesity class II (ARR, 1.28 [95% CI, 1.07-1.54]), and obesity class III (ARR, 1.24 [95% CI, 1.03-1.48]). Inadequate GWG was associated with increased SMM predominantly among individuals with normal weight (ARR, 1.13 [95% CI, 1.06-1.21]) and overweight (ARR, 1.19 [95% CI, 1.05-1.35]). GWG showed nonlinear associations with SMM across all BMI categories, with risk elevated at both low and high GWG in individuals with underweight, normal weight, and overweight, while in those with obesity classes I to III, risk was concentrated at high GWG values. Estimated SMM rates were lowest at 0.4 kg/wk GWG for underweight through overweight BMI categories and declined across obesity classes to 0.1 kg/wk GWG for the obesity class III category. Conclusions and Relevance In this cohort study of 1 035 927 pregnancies, GWG outside Institute of Medicine–recommended ranges was associated with increased SMM risk, with patterns varying by BMI category. These findings may complement existing evidence informing GWG guidance.