Maternal Exposure to Nitrate through Drinking Water as a Risk Factor for Congenital Anomalies

Background: Nitrate is a common water contaminant that has been associated with adverse birth outcomes, including certain birth defects, although evidence is limited. The purpose of this study was to examine whether maternal consumption of nitrate through drinking water is associated with an increased risk of congenital anomalies. Methods: The study included a total of 348,250 singletons births from the state of Missouri between January 1, 2004 and December 31, 2008. Individual-level birth defect data and maternal and child characteristics were obtained from the Missouri birth defects registry and state vital statistics records. Outcomes were linked with county-specific monthly estimates of the nitrate concentration in finished water, based on data collected for compliance with the Safe Drinking Water Standard. Poisson models were fit to examine the association between nitrate exposure and birth defects. Sensitivity analyses included restriction of the sample to counties with < 20% and < 10% private well usage to reduce exposure misclassification as well as limiting the analyses to residents of rural counties only to account for potential confounding by urbanicity. Results: Estimated water concentrations of nitrate (mean=0.37 mg/L) were generally low and all and county-level monthly mean estimates were below the Environmental Protection Agency’s maximum contaminant level of 10 mg/L. Nitrate exposure was associated with a significantly increased risk of limb deficiencies (aRR =1.26, 95% CI = 1.05, 1.51) in models without well restriction. Nitrate exposure was additionally weakly associated with an increased risk of congenital heart defects aRR = 1.13, 95%CI = 0.93, 1.38) and neural tube defects aRR = 1.18, 95%CI = 0.93, 1.51) in models with well restriction (< 10%). Conclusion: The results of this study should be interpreted with caution given the ecologic nature of the exposure estimates and limited information on other risk factors for congenital abnormalities. However, the positive associations found between nitrate exposure via drinking water and limb deficiencies, congenital heart defects, and neural tube defects are consistent with earlier epidemiologic studies. Our findings, if causal, may have serious policy implications given that exposure levels in our study were below current EPA standards for nitrate in drinking water.