Impact of Low Maternal Weight on Pregnancy and Neonatal Outcomes.

Objective: To examine the effect of underweight maternal body mass index (BMI) on pregnancy complications and neonatal outcomes.

Design: Cohort study.

Setting: Tertiary academic center.

Patients: A total of 16 361 mothers who delivered a singleton between 2015-2021 with either a BMI <18.5 kg/m² (n = 732) or normal BMI (18.5 ≥ BMI <23 or 25 kg/m², n = 15 629) at the initial prenatal visit or within 6 months of the initial visit.

Main outcome measures: Birthweight, gestational age, neonatal intensive care unit admission, preterm birth, and fetal death; obstetrical complications including preeclampsia/eclampsia, premature rupture of membranes, preterm premature rupture of membranes, and postpartum hemorrhage.

Results: Underweight women were younger and less likely to have private insurance (P < .01 for both) than normal-weight women. Approximately 23% of infants born to underweight mothers were small for gestational age and 15% were low birth weight vs 13.5% and 9% of infants of normal-weight mothers, respectively (P < .01 for both). These differences remained significant after adjusting for potential confounders. In adjusted logistic regression models, underweight women had a decreased risk of premature rupture of membranes and postpartum hemorrhage compared to normal-weight women.

Conclusion: Underweight BMI during pregnancy is associated with an increased risk of small for gestational age and low birth weight infants and a decreased risk of premature rupture of membranes and postpartum hemorrhage. These findings suggest underweight BMI during pregnancy increases the risk of adverse neonatal outcomes, while maternal-related pregnancy outcomes are less affected.