Prenatal exposure to nitrates associated with increased risk of preterm birth

Even low levels of nitrate-a common agricultural runoff and drinking water contaminant-are associated with increased risks of preterm birth and low birthweight babies, according to a new study published June 25 in PLOS Water by Jason Semprini of Des Moines University College of Health Sciences, US.

Nitrate is a naturally occurring compound increasingly found in inorganic fertilizers and, through runoff, in groundwater. When consumed by humans, nitrates can interfere with the blood's capacity to carry oxygen. Since 1992, the U.S. Environmental Protection Agency (EPA) has a maximum contaminant level for water-based nitrate at 10mg/L.

To evaluate possible reproductive risks, Semprini analyzed 357,741 birth records from Iowa spanning 1970 to 1988, linking each birth to county-level nitrate measurements taken within 30 days of conception. During the study period, nitrate levels rose an average of 8% per year, with a mean exposure of 4.2 mg/L.

After controlling for maternal and paternal characteristics as well as seasonal variation, the study found that early prenatal exposure to greater than 0.1mg/L nitrate-just 1% of the current EPA limit-was associated with an increase in preterm birth (Est.=+0.66%-points; C.I.=0.31, 1.01) and early prenatal exposure to greater than 5mg/L nitrate was associated with an increased risk of low birth weight babies (Est.=+0.33%-points; C.I.=0.03, 0.63). There were no additional risks conferred by exposure to elevated levels of nitrate, above 10 mg/L.

The author points out that the data is limited in scope, focusing only on one state. More recent data, as well as data from additional states and sources, such as private well water, could improve the strength of the association. 

"Our work adds to the evidence base that the current regulatory threshold (> 10mg/L) may be insufficient for protecting the in utero transmission of water-based nitrate during the first trimester of pregnancy," Semprini says. He concludes that current standards, unchanged since 1992, may need to be revisited as nitrate levels in groundwater continue to rise across the U.S.

He adds: "There is no safe level of prenatal nitrate exposure," and goes on to state: "[t]he estimated impact from prenatal exposure to nitrates reflects 15% of the harm from prenatal exposure to smoking cigarettes. I do not want to diminish the importance of efforts to prevent smoking during pregnancy...but, I must ask, do we give nitrates 15% of the attention we give to smoking?"

Semprini also notes: "The regulatory threshold for nitrates in public water does not consider prenatal exposure and has not been updated since established in 1992. Ignoring the potential harm from lower levels of prenatal nitrate exposure, the current regulatory standards are not adequately protecting America's mothers or children."