Babies born to mothers who live in areas near shale gas drilling wells are more likely to suffer birth defects, including congenital problems with their hearts and neural tube, a new study conducted by the Colorado School of Public Health warns.

Researchers looked at the rate of occurrence of birth defects among almost 125,000 babies born between 1996 and 2009 in rural Colorado, specifically in towns with a population under 50,000, and compared that data to how close their mothers lived to fracking wells. Results showed that children born to mothers who live within areas with the highest concentration of wells — at least 125 wells per mile — were twice as likely to develop neural tube defects as babies whose mothers lived in areas with no wells within a 10-mile radius. These babies also had 38% higher risk of developing congenital heart defects, researchers found.

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The rates of other birth defects were also examined in the study, including oral cleft defects, as well as preterm births and lower birth weight, but there was no evidence that the vicinity of drilling wells played any role in their occurrence.

Although the study does not prove any cause-effect relation between birth defects and fracking wells, its authors noted that further research should be carried out to examine the full implications of natural gas development on public health.