Public Water Arsenic and Birth Outcomes in the Environmental Influences on Child Health Outcomes Cohort.

Importance: Inorganic arsenic is associated with adverse birth outcomes, but evidence is limited for public water concentrations (modifiable by federal regulatory action) in US populations. Objective: To evaluate the association between prenatal public water arsenic exposure below the federal regulatory standard of 10 μg/L and birth outcomes in the US. Design, Setting, and Participants: This cohort study analyzed observational pregnancy cohort data from the Environmental Influences on Child Health Outcomes (ECHO) Cohort for birthing parent-infant dyads from 35 pregnancy cohort sites. Infants were born between 2005 and 2020. The data were analyzed between 2024 and 2025. Exposure: Individual, time-weighted, mean prenatal public water arsenic exposures were estimated by joining Zip Code Tabulation Area-level public water arsenic concentrations with monthly residential history data during pregnancy. Main Outcome and Measure: Adjusted risk ratios (RRs) of preterm birth, low birth weight, and small for gestational age were evaluated. Adjusted RRs, mean differences in birth weight-for-gestational age z score and birth weight, and the geometric mean ratio of gestational age at birth were calculated via cubic splines, per 1 μg/L higher prenatal water arsenic, and across policy-relevant categories of exposure. Results: The cohort comprised 13 998 birthing parents (mean [SD] age, 30.8 [5.6] years) of whom 4.5% were of American Indian, Alaska Native, Native Hawaiian, or Pacific Islander; 7.2% Asian; 12.4% Black; 56.1% White; 4.2% multiple races; and 8.5% another race and 28.1% were of Hispanic/Latino and 70.4% non-Hispanic/Latino ethnicity. Prenatal public water arsenic ranged from less than 0.35 to 37.28 μg/L. In spline models, prenatal public water arsenic was associated with a higher risk of low birth weight, lower birth weight, and lower birth weight-for-gestational age z score, although effect estimates lacked precision. The RR of low birth weight per 1 μg/L higher prenatal water arsenic was higher among Black (1.02; 95% CI, 1.01-1.03), Hispanic/Latino (1.07; 95% CI 1.02-l.12), and White (1.04; 95% CI, 102-1.06) birthing parents, and the RR for preterm birth was higher among Hispanic/Latino birthing parents (1.05; 95% CI, 1.01-1.09). The mean difference of birth weight and birth weight-for-gestational age z score per 1 μg/L higher prenatal water arsenic was more pronounced among White birthing parents (-10 g [95% CI, -17 to -3 g]; -0.02 SDs [95% CI -0.03 to -0.01 SDs]). No evidence that prenatal public water arsenic mediated the association between birthing parent race and ethnicity and adverse birth outcomes was observed. Conclusions and Relevance: In this cohort study of birthing parent-infant dyads across the US, arsenic measured in public water systems was associated with birth outcomes at levels below the current US Environmental Protection Agency's maximum contaminant level. The findings suggest that further reducing the maximum contaminant level for arsenic may decrease the number of infants with low birth weight in the US.