The association between social vulnerability and racial disparities in preterm birth

To evaluate the association between county-level social vulnerability and preterm birth and assess whether racial disparities vary across levels of vulnerability.

This cross-sectional study utilized restricted Centers for Disease Control and Prevention natality data from 2023. We included Black or White births with singleton pregnancies across 3123 US counties from 50 US states and the District of Columbia. Participants were grouped into quartiles based on the county-level Social Vulnerability Index (SVI), with quartile 4 representing the most socially vulnerable populations. The primary outcome was preterm birth, defined as delivery before 37 weeks. Secondary outcomes included small for gestational age (SGA; birth weight <10th percentile) and hypertensive disorders of pregnancy (HDP). We used mixed-effects generalized linear models with Poisson distribution with robust variance to calculate average marginal effects (AMEs) with 95% confidence intervals (95% CIs), adjusted for confounders. Difference-in-differences (DID) was calculated for the difference in Black-White disparity across SVI quartiles, with quartile 1 as a reference.

A total of 2,122,789 births were included: 355,774 in quartile 1; 535,907 in quartile 2; 611,074 in quartile 3; and 620,034 in quartile 4. Black births had higher adjusted rates of preterm birth than White births in all quartiles. Black and White preterm birth rates were 9.8% versus 7.8% (AME, 2.0%; 95% CI, 1.6-2.5) in quartile 1; 11.0% versus 7.9% (AME, 3.1%; 95% CI, 2.7-3.4) in quartile 2; 11.3% versus 8.3% (AME, 3.0; 95% CI, 2.-3.3) in quartile 3; and 12.2% versus 8.6% (AME, 3.7%; 95% CI, 3.4-4.0) in quartile 4. Racial disparities increased with higher social vulnerability; DID was statistically significant in quartiles 2-4 (1.1% [95% CI, 0.5-1.6] in quartile 2, 1.0% [95% CI, 0.4-1.6] in quartile 3, and 1.6% [95% CI, 1.1-2.2] in quartile 4 when compared to quartile 1). Similar patterns were observed for SGA and HDP.

Increasing social vulnerability was associated with widening racial disparities in preterm birth and other adverse pregnancy outcomes. These findings underscore the need to address community-level influences on perinatal inequities.