Evaluation of nitrates in drinking water, maternal methemoglobin, and complications of pregnancy in a rural population

Pregnant women are considered vulnerable to the effects of exposure to high levels of nitrates in drinking water due to their altered physiological states. There are also data indicating that pregnancy complications are associated with reduced antioxidant activity during pregnancy, which can be exacerbated by exogenous oxidants such as nitrates.This prospective study investigated changes in methemoglobin levels during pregnancy and the relationship to factors that affected those changes, including nitrates in drinking water. In a nested case-control design, this study also explored the association between methemoglobin levels, antioxidant status and the most frequent pregnancy complications. A total of 357 women were included in the investigation, and 191 in the sub-analyses of pregnancy complications. Longitudinal regression models were used to examine changes in methemoglobin levels during pregnancy. Logistic regression was used to evaluate the association between pregnancy complications and antioxidant status.Pregnant women in this study showed a consistent decrease in methemoglobin levels as their pregnancy progressed, regardless of the source of their drinking water or the level of nitrates in their tap water. Pregnant women with lower nitrate levels in their water showed lower methemoglobin levels as pregnancy progressed (betaeta= -1.341, p<0.0001). Methemoglobin levels in newborns were correlated with maternal levels at birth, and infant's levels decreased over time. Selected pregnancy complications were associated with decreased concentrations of the antioxidant glutathione in the first trimester of pregnancy (Odds Ratio and 95% Confidence Interval: 0.08 0.004-1.6). At the exposure levels documented in this study, no association was seen between methemoglobin levels, nitrate levels in drinking water and glutathione antioxidant status. This study suggested that: (1) pregnant women potentially exposed to nitrate levels primarily below the maximum contaminant level for drinking water were unlikely to show increased methemoglobin levels, and (2) water use practices (such as the use of treatment devices to remove nitrates) varied according to water source and should be considered in the assessment of exposure. Another implication of this study was that women who developed pregnancy complications appeared more likely to have had a lower antioxidant reserve in early pregnancy.