Mediating effect of social determinants of health between race and phthalate biomarkers in pregnancy.

Intro: Prenatal phthalate exposure is linked to adverse reproductive and developmental outcomes and is consistently higher among non-Hispanic Black compared to non-Hispanic White pregnant individuals, yet the factors responsible for these disparities are poorly characterized. We sought to quantify Black-White differences in repeated urinary concentrations of phthalate biomarkers during pregnancy and to estimate the extent to which neighborhood-level social determinants of health (SDoH) mediate these disparities. Methods: This secondary analysis used data from the Human Placenta Project Study, a prospective observational cohort study conducted at two academic institutions between 2016 and 2018. Eligible participants had singleton pregnancies, were recruited before 14 gestational weeks, and self-identified as Black or White; individuals identifying as Hispanic were excluded. Urine specimens were collected at up to 8 visits (12-38 weeks' gestation). Statistical analyses were performed from January to April 2025. Our exposures included self-identified race (Black vs White) and neighborhood SDoH, principally the 2018 Social Vulnerability Index (SVI) and the National Walkability Index, linked by geocoded residential address. Specific-gravity-adjusted concentrations of 14 phthalate metabolites and 4 replacement metabolites were quantified by high-performance liquid chromatography-tandem mass spectrometry. We used Bayesian multivariate mediation models to estimate adjusted geometric mean ratios (GMRs) with 95% credible intervals (95% CrI) for the total effect of race and the joint indirect effects via SVI and Walkability, and we reported the proportion mediated. Results: Among 249 participants (mean age, 26.5 years; 131 [52.6%] Black), 1562 urine samples were analyzed. Black participants had higher pregnancy-average geometric means of mono-ethyl phthalate (MEP; total GMR, 3.87; 95% CrI, 3.03-4.93), Σdi-n-butyl phthalate (ΣDnBP; 1.86; 1.52-2.27), and mono-benzyl phthalate (MBzP; total GMR, 1.41; 95% CrI, 1.04-1.90). The proportion mediated for SVI and Walkability jointly was 10% for MEP, 18% for ΣDnBP, and 44% for MBzP. No significant racial differences were observed for other phthalate biomarkers. Conclusions: Black pregnant individuals had higher phthalate biomarker concentrations for several phthalates, and up to nearly half of the disparity for MBzP was explained by neighborhood-level SDoH. Interventions aimed at improving community social and built environments may help reduce racially patterned chemical exposures during pregnancy.