Exploring non-linear associations of maternal pre-pregnant body mass index with risk of stillbirth, infant and neonatal mortality in over 21 million US births

Background Higher maternal pre-pregnancy body mass index (BMI) has been associated with higher risk of stillbirth, infant and neonatal mortality. Few studies have explored associations of underweight, with those that have varying in their conclusions. Our aim was to examine the risk of stillbirth, infant and neonatal mortality across the pre-pregnancy BMI distribution and establish a likely healthy BMI range. Methods We used publicly available birth, infant death and fetal death datasets from the US National Center for Health Statistics National Vital Statistics System, 2014–2020. Fractional polynomial multivariable logistic regression was used to examine the nature of associations between maternal pre-pregnant BMI and stillbirth (birth with no signs of life at ≥24 weeks), infant mortality (death of a live born baby aged <365 days) and neonatal mortality (death of a live born baby aged <28 days). Findings There were 56,376/21,437,556 (0.26%) stillbirths, 108,413/24,742,273 (0.44%) infant deaths and 66,801/24,742,273 (0.27%) neonatal deaths among complete cases. Mean BMI was 27.0 kg/m 2 . We found non-linear associations between pre-pregnant BMI and all three outcomes - risk was elevated at both low and high BMIs although, for stillbirth, the increased risk at low BMI was less marked than for infant and neonatal mortality. The lowest risk was at a BMI of 21 kg/m 2 for infant and neonatal mortality and, for stillbirth, at 18 kg/m 2 . Interpretation Public health messaging for preconception and postnatal care should focus on healthy weight to maximise maternal and child health, and not focus solely on maternal overweight or obesity.