Impact of Low Maternal Weight on Pregnancy and Neonatal Outcomes

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

Cohort study.

Tertiary academic center.

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.

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.

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.

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.