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Background:

Nitrate is a widespread groundwater contaminant and a leading cause of drinking water quality violations in California. Associations between nitrate exposure and select adverse birth outcomes have been suggested, but few studies have examined gestational exposures to nitrate and risk of preterm birth (before 37 wk gestation).

Objective:

We investigated the association between elevated nitrate in drinking water and spontaneous preterm birth through a within-mother retrospective cohort study of births in California.

Methods:

We acquired over 6 million birth certificate records linked with Office of Statewide Health Planning and Development hospital discharge data for California births from 2000–2011. We used public water system monitoring records to estimate nitrate concentrations in drinking water for each woman’s residence during gestation. After exclusions, we constructed a sample of 1,443,318 consecutive sibling births in order to conduct a within-mother analysis. We used separate conditional logistic regression models to estimate the odds of preterm birth at 20–31 and 32–36 wk, respectively, among women whose nitrate exposure changed between consecutive pregnancies.

Results:

Spontaneous preterm birth at 20–31 wk was increased in association with tap water nitrate concentrations during pregnancy of 5 to $<10\mathrm{mg}/\mathrm{L}$ [odds ratio $\left(\text{OR}\right)=1.47$; 95% confidence interval (CI): 1.29, 1.67] and $\ge 10\mathrm{mg}/\mathrm{L}$ ($\text{OR}=2.52$; 95% CI: 1.49, 4.26) compared with $<5\mathrm{mg}/\mathrm{L}$ (as nitrogen). Corresponding estimates for spontaneous preterm birth at 32–36 wk were positive but close to the null for 5 to $<10\mathrm{mg}/\mathrm{L}$ nitrate ($\text{OR}=1.08$; 95% CI: 1.02, 1.15) and for $\ge 10\mathrm{mg}/\mathrm{L}$ nitrate ($\text{OR}=1.05$; 95% CI: 0.85, 1.31) vs. $<5\mathrm{mg}/\mathrm{L}$ nitrate. Our findings were similar in several secondary and sensitivity analyses, including in a conventional individual-level design.

Discussion:

The results suggest that nitrate in drinking water is associated with increased odds of spontaneous preterm birth. Notably, we estimated modestly increased odds associated with tap water nitrate concentrations of 5 to $<10\mathrm{mg}/\mathrm{L}$ (below the federal drinking water standard of $10\mathrm{mg}/\mathrm{L}$) relative to $<5\mathrm{mg}/\mathrm{L}$. https://doi.org/10.1289/EHP8205