Abstract
Background/Objectives: In the United States, Black infants are twice as likely as infants of all other races and ethnicities to die by one year of age. Mediation modeling predicted that preventing low birthweight could alleviate 75% of this disparity. However, the potential confounding and interacting role of preterm birth remains a question. The goal of this study was to determine how birthweight and length of gestation interact in causing racial disparity. Methods: Records from more than 25 million singleton births were retrieved from the United States National Natality Database for the years 2016 to 2022. Two interaction models were evaluated using Bayesian estimation of potential outcomes. The first modeled the interaction between birthweight and length of gestation with both mediators measured as binary (normal/abnormal). The second modeled the interaction using five classifications for both birthweight and length of gestation. Results: Eliminating either abnormal birthweights or abnormal lengths of gestation would reduce racial disparity in infant mortality by approximately 75%. There was no additional reduction of racial disparity by normalizing both. Modeling the combinations of specific categories of birthweight and length of gestation showed Black infants were 2.76 (2.72, 2.79) times more likely to be born with extremely low birthweight and extremely preterm delivery. This single combination explained over 60% of the racial disparity in infant mortality. Conclusions: The current study clarifies how birthweight and preterm birth contribute to racial disparity and illustrates how Bayesian estimation of potential outcomes enables complex mediational investigations.