Early prenatal nitrate exposure and birth outcomes: A study of Iowa’s public drinking water (1970–1988)

Despite the biological mechanisms linking prenatal nitrate exposure to birth outcomes, epidemiological research has been inconclusive. The evidence-base has been limited by where and how nitrate exposure was measured, and the spurious correlation between geotemporal nitrate heterogeneity and unmeasurable factors contributing to gestational age and birth weight. We linked Iowa water quality data and birth records to estimate the independent association between early prenatal nitrate exposure and birth outcomes. Accessing Community Water Supply Quality Data, we calculated the median nitrate (mg/L) level for each county-date. With birth certificate microdata from the National Center for Health Statistics, we linked every Iowa birth (1970–1988) to a county-level nitrate measure within thirty days of conception. The outcomes were gestational age (weeks), preterm birth ( 10 mg/L, > 5 mg/L, > 0.1 mg/L, > 0.0 mg/L). We constructed linear regression models which controlled for maternal and paternal characteristics, and county-year and year-month fixed-effects to account for unobservable annual variation between counties and longitudinal variation within all counties. Among 357,741 births, mean nitrate exposure was 4.2 mg/L. Early prenatal exposure to >0.1 mg/L nitrate was associated preterm birth (Est. = +0.66%-points; C.I. = 0.31, 1.01). Early prenatal exposure to 5 mg/L nitrate was associated with low birth weight (Est. = +0.33%-points; C.I. = 0.03, 0.63). The associations between elevated exposure to nitrate and any birth outcomes did not differ from lower levels of exposure. Prenatal exposure to nitrate below the > 10 mg/L standard may cause harm. Since establishing this standard in 1992, groundwater nitrate levels have risen. Our results warrant greater scholarly and policymaking attention to understand and combat the adverse effects of nitrate.Author summary: Rising levels of nitrate in our drinking water pose a threat to public health. Emerging evidence suggests a possible association between early prenatal exposure to nitrate in drinking water and adverse birth outcomes. Understanding this association has proved challenging, as there is currently no regulatory guidance on safe levels of nitrate during the early prenatal period and many factors contribute to adverse birth outcomes. Focusing on Iowa, a state with some of the highest levels of ground water nitrate in the country, this study linked public water quality data to more than 350,000 birth records. During the study period (1970–1988), nitrate levels increased 8% annually. Mean nitrate levels were 4.2 mg/L, well below the 10 mg/L regulatory threshold. Interestingly, exposure to nitrate above the 10 mg/L threshold had no association with birth outcomes. Rather, the study showed that prenatal exposure to lower levels of nitrate were adversely associated with gestational age and low birthweight. These results suggest that current regulatory standards, which have not been updated since 1992, may be inadequate.