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Purpose: Previous studies have reported an association between chronic exposure to elevated levels of drinking water arsenic and cardiovascular and other health effects. There is debate, however, whether human health effects exist at lower exposure levels. Prior studies have been unable to investigate these potential health effects due to imprecise exposure estimation. Examining a sensitive subpopulation such as pregnant women proves more efficient in detecting potential health effects associated with drinking water arsenic. Methods: A population-based cohort study was conducted based on the prenatal care visits of 5306 women in Inner Mongolia, China, from 1997 to 1999. Repeated measures mixed models were used to assess associations between drinking water arsenic estimates and serial blood pressure measurements taken during prenatal care visits. Results: Once selected confounders were controlled for, exposure to drinking water arsenic was positively associated with mean arterial blood pressure, even at lower ranges of exposure. Arsenic exposure categories of 51 to 100 μg/L and more than 100 μg/L were associated with increases in mean arterial pressure of 3.56 (95% CI=1.66, 5.47) and 2.30 (0.57, 4.02) mmHg, respectively, relative to the mean arterial pressure of women exposed to drinking water arsenic concentrations ranging from below detectable levels to 20 μg/L. These changes were not consistent over the course of gestation and became somewhat less pronounced later in pregnancy. Conclusion: Mean arterial pressure during pregnancy is associated with exposure to drinking water arsenic among otherwise healthy pregnant women, suggesting a potential vascular effect of drinking water arsenic at lower doses than had been previously reported. [Copyright &y& Elsevier]