Living near farms that use the weed killer atrazine may up the risk of a rare birth defect, according to a study presented this past Friday at the annual meeting of the Society for Maternal-Fetal Medicine in Chicago.

About 1 in 5000 babies born in the U.S. each year suffers from gastroschisis, in which part of the intestines bulges through a separation in the belly, according to the March of Dimes. The rate of gastroschisis has risen 2- to 4-fold over the last three decades, according to Dr. Sarah Waller, of the University of Washington, Seattle, and colleagues.

Waller’s team studied the potential link between the weed killer and the birth defect because, as they note in their conference abstract, “during the last 10 years, the highest percentage per population of gastroschisis was in Yakima County, in the eastern part of the state, where agriculture is the primary industry.”

Overall, Washington state has about double the national average of gastroschisis cases – an average of 43 cases per year, Waller told Reuters Health.

The researchers looked at more than 4,400 birth certificates from 1987-2006 – including more than 800 cases of gastroschisis — and U.S. Geological Survey databases of agricultural spraying between 2001 and 2006.

Using Environmental Protection Agency (EPA) standards to define high chemical exposure levels in surface water, they found that the closer a mother lived to a site of high surface water contamination by atrazine, the more likely she was to deliver an infant with gastroschisis.

The birth defect occurred more often among infants who lived less than 25 km (about 15 miles) from one of these sites, and it occurred more often among babies conceived between March and May, when agricultural spraying is common.

A mother’s tobacco use, and being the first born, were also linked to a higher rate of the birth defect. Two other commonly used farm chemicals – nitrates and 2,4-dichlorophenoxyacetic acid (2,4-D) – were not linked to gastroschisis.