Trends in US Preterm Birth Rates by Household Income and Race and Ethnicity.

Importance: There are well-documented racial and ethnic disparities in preterm birth in the US. The role of household income in temporal preterm birth trends remains largely unexplored. Objective: To examine US preterm birth trends by household income from 2011 to 2021, as well as associations between income and preterm birth according to race and ethnicity. Design, Setting, and Participants: This US population-based, cross-sectional study used data on 411 469 mothers of infants aged 2 to 4 months from the nationally representative Pregnancy Risk Assessment Monitoring System database from 2011 to 2021. Data were analyzed from January to April 2024. Exposures: Mothers reported annual household income, which was categorized as less than 100% of the federal poverty level (FPL), 100% to 199% of the FPL, and 200% or more of the FPL based on year, state, and household size. Main Outcome and Measures: The main outcome was preterm birth, defined as birth at less than 37 weeks' gestation. Maternal self-reported race and ethnicity was defined as American Indian or Alaska Native, Asian, Hispanic (any race), non-Hispanic Black, non-Hispanic White, and other or multiracial. Trends in preterm birth by income categories were examined, and modified Poisson regression models were built to (1) examine the association between income and preterm birth, (2) adjust for sociodemographic and pregnancy-related covariates, (3) adjust additionally for race and ethnicity, and (4) introduce an interaction between race and ethnicity and income. Results: Among 411 469 (weighted 20 million) mother-infant dyads (0.8% American Indian or Alaska Native, 5.5% Asian, 15.5% Hispanic, 14.1% non-Hispanic Black, 58.9% non-Hispanic White, and 3.1% other or multiracial), rates of preterm birth increased significantly over time in the groups reporting an annual household income of less than 100% of the FPL (2011, 9.7%; 2021, 11.1%) and 100% to 199% of the FPL (2011, 7.8%; 2021, 10.0%). The preterm birth rate was highest among households reporting household income less than 100% of the FPL within all racial and ethnic groups except Asian. Non-Hispanic Black mothers had the highest rates of preterm birth across all income categories. The association of income with preterm birth remained significant after adjusting for covariates but attenuated to the null after including race and ethnicity in the model. In the lowest income group, non-Hispanic Black mothers had a 19% greater risk of preterm birth compared with non-Hispanic White mothers (adjusted relative risk [ARR], 1.19; 95% CI, 1.11-1.27), whereas in the highest income group, non-Hispanic Black mothers had a 13% greater risk of preterm birth compared with non-Hispanic White mothers (ARR, 1.13; 95% CI, 1.01-1.26). Conclusions: In this population-based cross-sectional study, household income disparities in preterm birth widened over time. Black race moderated the association between income and preterm birth, underscoring the need to examine the role of racism in preterm birth disparities.