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

## TL;DR

Preterm birth rates increased among low-income US mothers, but Black mothers had the highest rates regardless of income, suggesting income-focused interventions may not be enough to address racial disparities.

## Contribution

The study reveals that racial disparities in preterm birth persist across income levels, highlighting the need to address systemic factors like racism.

## Key findings

- Preterm birth rates increased among low-income households but remained stable for higher-income households.
- Non-Hispanic Black mothers had the highest preterm birth rates across all income levels.
- Adjusting for race and ethnicity reduced the income-preterm birth association, suggesting race plays a significant role.

## Abstract

How have US preterm birth trends varied by household income over time, and to what extent are race and ethnicity associated with this variance?

In this cross-sectional study of 411 469 births using 2011-2021 Pregnancy Risk Assessment Monitoring System data, preterm birth rates increased among low-income households (<200% of the federal poverty level) but remained stable among higher-income households. Race moderated the association between income and preterm birth; Black mothers had the highest preterm birth rates across all income levels.

In this study, racial and ethnic disparities in the preterm birth rate persisted, suggesting that interventions addressing income alone may be insufficient to reduce inequities in preterm birth.

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.

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.

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.

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.

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.

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).

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.

This cross-sectional study uses nationally representative data from the Pregnancy Risk Assessment Monitoring System database to examine temporal US preterm birth trends by income categories from 2011 to 2021, as well as associations between income and preterm birth according to race and ethnicity.