The impact of prepregnancy body mass index on pregnancy and neonatal outcomes

The obesity epidemic in the United States is continuing to worsen. Obesity is a known risk factor for pregnancy morbidity. However, many studies use the patient's body mass index (BMI) at the time of delivery, do not stratify by class of obesity, or utilize billing codes as the basis of their study, which are noted to be inaccurate.

This study aims to investigate the prepregnancy BMI class specific risks for pregnancy and neonatal complications based on a prepregnancy BMI class.

We conducted a retrospective cohort study of 40,256 pregnant women with 55,202 singleton births between October 16, 2007 and December 3, 2023. We assessed the risk of pregnancy and neonatal morbidity based on the maternal prepregnancy BMI category. The primary outcome was composite maternal morbidity, including hypertensive disorders of pregnancy (i.e., gestational hypertension [GHTN] and preeclampsia), and gestational diabetes mellitus (GDM), adjusted for pregestational diabetes mellitus and chronic hypertension (cHTN). Secondary maternal outcomes included preterm premature rupture of membranes (PPROM), preterm delivery (PTD<37 and <32 weeks), induction of labor (IOL), cesarean delivery (CD), and postpartum hemorrhage (PPH). Neonatal outcomes included a composite adverse outcome (including stillbirth, intraventricular hemorrhage (IVH), hypoglycemia, respiratory distress syndrome [RDS], APGAR [Appearance, Pulse, Grimace, Activity, and Respiration] <7 at 5 min, and neonatal intensive care unit [NICU] admission), birthweight, fetal growth restriction (FGR), and macrosomia.

Composite maternal morbidity (odds ratio [OR] 4.40, confidence interval [CI] 3.70-5.22 for class III obesity [BMI≥40.0 kg/m²] compared with normal BMI), hypertensive disorders of pregnancy (HDP), GDM, PTD, IOL, CD, PPH, neonatal composite morbidity, hypoglycemia, RDS, APGAR<7 at 5 min, NICU admission, and macrosomia showed a significant increasing test of trend among BMI classes. Increased BMI was protective for FGR.

Our data provides BMI-class specific odds ratios (ORs) for adverse pregnancy outcomes. Increased BMI class significantly increases the risk of HDP, GDM, IOL, CD, composite adverse neonatal outcomes, and macrosomia, and decreases the risk of FGR. Attaining a healthier BMI category prior to conception may lower pregnancy morbidity.