Maternal Pre-Pregnancy Body Mass Index and Risk of Preterm Birth: The Role of Weight Gain During Pregnancy, Race and Ethnicity.

Background: Preterm birth is one of the leading causes of infant and neonatal mortality. Pre-pregnancy body mass index (BMI) has been linked to preterm birth but the evidence of this association by weight gain during pregnancy, race, and ethnicity is limited. This study aimed to assess the association between maternal pre-pregnancy BMI and preterm birth stratified by weight gain during pregnancy, race, and ethnicity. Methods: The U.S. natality data from 2017 to 2021 was used. In this analysis, we included mothers who had a live singleton birth and available data for pre-pregnancy BMI, gestational age at birth, weight gain during pregnancy, race, and ethnicity. Logistic regression models were used to assess the association between pre-pregnancy BMI categories and preterm birth stratified by weight gain during pregnancy, race, and ethnicity. Results: A total of 17,311,509 singleton live births were included of which 1,393,889 (8.05 %) were preterm births. After adjusting for confounders, compared to normal pre-pregnancy BMI mothers (18.5-24.9), those with underweight BMI (<18.5) were at increased odds of preterm birth regardless of weight gain during pregnancy, race, and ethnicity. However, for mothers with a pre-pregnancy BMI above the normal weight (≥25), the association between pre-pregnancy BMI and preterm birth differs by weight gain during pregnancy, race, and ethnicity. Asian mothers with obesity II (35.0-39.9) had 93% (OR=1.93, 95%:1.62-2.30) increased odds of preterm birth for weight gain during pregnancy of 31-40 pounds. Their Whites, Hispanic and Blacks counterparts experienced lower odds of preterm birth for similar weight gain during pregnancy (OR=1.56, 95%:1.51-1.60), (OR=1.48, 95%: 1.41, 1.54) and (OR=1.22, 95%:1.17-1.27), respectively. Conclusion: Mothers with underweight BMI were at increased risk of preterm birth regardless of weight gain during pregnancy, race, and ethnicity. However, the association between high pre-pregnancy BMI and preterm birth varied by weight gain during pregnancy, race, and ethnicity.