Impact of pre-pregnancy body mass index in low-risk pregnancies with adequate gestational weight gain: trend and adverse outcomes

OBJECTIVE: To examine the temporal trend of pre-pregnancy body mass index (BMI) and the association between pre-pregnancy BMI and adverse outcomes in low-risk pregnancies with adequate gestational weight gain (GWG). STUDY DESIGN: We conducted a retrospective cohort study using U.S. vital statistic data (2016-2023). We included low-risk pregnancies with adequate GWG that delivered non-anomalous, singleton live births at 37 to 41 weeks of gestation in a hospital; those with diabetes or hypertensive disorders were excluded. The exposure variable was pre-pregnancy BMI. Adverse outcomes were examined. The average annual percent change (AAPC) was calculated to assess the trend of pre-pregnancy BMI. Multivariable Poisson regression models with robust error variance were used to examine the association. Adjusted relative risks (aRR) with 95% confidence intervals (CI) were calculated. RESULTS: Of 29,972,027 live births during the study period, 6,741,787 (22.5%) were included; among them, pre-pregnancy BMI distribution was as follows: 4.4% underweight, 52.9% normal weight, 23.3% overweight, and 19.4% obese. Over the 8 years, there was a significantly increased trend in the prevalence of maternal pre-pregnancy overweight (AAPC=2.6; 95% CI 2.1 to 3.2) and obesity (AAPC=3.6; 95% CI 2.9 to 4.4) in low-risk pregnancies with adequate GWG. Compared to women with pre-pregnancy normal weight, those with pre-pregnancy obesity were associated with a higher risk of primary cesarean delivery (aRR=1.57, 95% CI=1.56-1.58). Importantly, those with pre-pregnancy obesity class III were associated with the highest risk of primary cesarean delivery (aRR=2.01, 95% CI=1.99-2.02). Similar results were also noted in 5-minute Apgar <5, assisted ventilation >6 hours, neonatal seizure, neonatal ICU, neonatal death and infant death. CONCLUSIONS: Among low-risk pregnancies with adequate GWG, there was an upward trend in pre-pregnancy overweight and obesity from 2016 to 2023. The risks of adverse outcomes were the highest among pregnancies with pre-pregnancy obesity, especially with class III obesity.