Neonatal and Maternal Outcomes in Nulliparous Individuals according to Prepregnancy Body Mass Index

Objective This study aimed to examine the effect of incremental changes in body mass index (BMI, kg/m(2)) on the association with adverse pregnancy outcomes. Study Design This was a retrospective cohort study of U.S. vital statistics Live Birth and Infant Death linked data from 2011 to 2020. We limited analyses to nulliparas with singleton pregnancies who delivered at 20 weeks or greater. Outcomes were compared according to the prepregnancy BMI category using 5 kg/m(2) increments, with each of the other BMI categories sequentially as the referent. The composite neonatal outcome was defined as any neonatal death, neonatal intensive care unit (ICU), surfactant use, ventilation use, or seizure. Severe maternal morbidity was defined as any maternal ICU, transfusion, uterine rupture, and hysterectomy. Adjusted relative risks were calculated for each BMI category as a referent group, using modified Poisson regression and adjusting for confounders. Results A total of 11,174,890 nulliparous individuals were included. From 2011 to 2020, the proportions of individuals with BMI 40 or greater, BMI 50 or greater, and BMI 60 or greater increased significantly (from 3.1 to 4.9%, from 0.4 to 0.6%, from 0.03 to 0.06%, respectively; all trend p-values < 0.001). As BMI deviated from normal BMI, risks of neonatal and maternal adverse outcomes increased progressively. For example, as BMI deviated from normal BMI (18.5-24.9), the risk of composite neonatal outcome increased by 2% in individuals with BMI < 18.5 and up to 2.11-fold in individuals with BMI 65-69.9. When compared with BMI 40 to 44.9, BMI 35 to 39.9 was associated with an 8% decreased risk of composite neonatal outcome, whereas BMI 45 to 49.9 was associated with an 8% increased risk of composite neonatal outcome. Conclusion Incremental increases in prepregnancy BMI are linked to higher risks of adverse pregnancy outcomes, highlighting the need for effective weight management before conception.