Pregnancy weight gain was associated with severe maternal morbidity across BMI groups

What happened

A retrospective cohort study of 1,035,927 singleton pregnancies in Ontario found that gestational weight gain above 2009 Institute of Medicine ranges was associated with severe maternal morbidity across every prepregnancy BMI category. Inadequate gain was also associated with higher risk among people classified as normal weight or overweight. The study identifies population-level associations and cannot determine that changing one person's weight gain would prevent a complication.

The association appeared across the BMI spectrum

Researchers linked Ontario birth-registry and hospital data for singleton deliveries at 20 weeks or later between April 2012 and March 2022. Severe maternal morbidity was defined as a composite of 13 serious conditions occurring during the delivery hospitalization. Analyses accounted for factors including age, parity, area-level socioeconomic measures, smoking, fetal sex and several pre-existing medical conditions.

Compared with gestational weight gain inside the guideline range, excessive gain was associated with adjusted rate ratios between 1.16 and 1.33 across the six prepregnancy BMI categories. Inadequate gain was associated with higher morbidity in the normal-weight and overweight groups. The estimated weekly gain associated with the lowest modelled risk decreased as prepregnancy BMI increased, but risk curves were relatively flat across a range of gains in the obesity groups.

The study should not become a rule for individual pregnancies

Because this was an observational analysis, it cannot show that weight gain itself caused the recorded complications. Weight change can reflect fluid retention, illness, fetal growth, nutrition, treatment or other factors that also influence risk. Some associations weakened when the researchers restricted analyses to term pregnancies or removed hypertensive disorders from the composite outcome.

The dataset captured severe morbidity during delivery hospitalization but not events recorded only during an earlier pregnancy admission or a later postpartum readmission. Prepregnancy weight may have been self-reported for some participants, individual conditions were too rare to model separately and the broad composite joins outcomes with different biological pathways. The results may inform future guideline evaluation; they are not a reason for restrictive dieting or unsupervised weight change during pregnancy.

Why it matters

Pregnancy guidance needs to balance fetal growth, maternal health, nutrition and the wide variation between individual bodies. A cohort this large can reveal patterns that smaller studies miss, but responsible reporting must avoid turning an association into blame or a universal prescription.

What to watch

Replication in other health systems, analyses of specific morbidity outcomes and postpartum events, prospective research on weight trajectories, and whether future guidelines incorporate absolute risk and clinical context. Pregnant people should discuss nutrition and weight concerns with their maternity-care team.