Pre-pregnancy body mass index and adverse maternal and perinatal outcomes in twins: A population retrospective cohort study.

OBJECTIVE: To examine the association between pre-pregnancy BMI and severe maternal morbidity (SMM), perinatal death and severe neonatal morbidity in twin pregnancies. METHODS: All twin births at ≥ 20 weeks gestation in British Columbia, Canada, from 2000 to 2017 were included. We estimated rates of SMM, a perinatal composite of death and severe morbidity, and its components per 10,000 pregnancies. Confounder-adjusted rate ratios (aRR) between pre-pregnancy BMI and outcomes were estimated using robust Poisson regression. RESULTS: Overall, 7770 (368 underweight, 1704 overweight, and 1016 obese) women with twin pregnancy were included. The rates of SMM were: 271.1, 320.4, 270.0, and 225.9 in underweight, normal BMI, overweight and obese women, respectively. There was little association between obesity and any of the primary outcomes (e.g., aRR = 1.09, 95% CI = 0.85, 1.38 for composite perinatal outcome). Underweight women had higher rates of the composite perinatal adverse outcome (aRR = 1.79, 95% CI = 1.32-2.43), driven by increased rates of severe respiratory distress syndrome, and neonatal death. CONCLUSIONS: There was no evidence of elevated risk of adverse outcomes among twin pregnancies of women who were overweight or obese. Risk was higher in underweight women, who may require specific care when carrying twins.