In his recent State of the Union address, President Barack Obama praised natural gas as “the bridge fuel that can power our economy with less of the carbon pollution that causes climate change” and vowed to “cut red tape” to help business invest in it. But two studies released this winter bolster long-held fears that the extraction process, hydraulic fracturing, or fracking, presents serious dangers for human health—and in particular, the health of the unborn.

One of the studies was conducted in Colorado, where some cities have sought a moratorium on fracking and industry has pushed back, by public health scientists from the Colorado School of Public Health and BrownUniversity. The central finding is a strong correlation between proximity to fracking wells and congenital heart defects. As the number and nearness of wells to a pregnant woman’s home went up, so did the likelihood that her baby would develop a heart problem. Strikingly, “Births to mothers in the most exposed tertile [an exposure level equal to 125 wells within mile of the home] had a 30% greater prevalence of CHDs [congenital heart defects]…than births to mothers with no wells within a 10-mile radius of their residence.”

The authors also saw some evidence that fracking wells upped the incidence of neurological defects, though only at high levels of exposure. They looked for a correlation with oral clefts, low birth weight, and premature birth, but did not find that fracking made them more likely.

A study in Pennsylvania, another state rich in natural gas, had different but worrisome findings. (Authored by researchers from Princeton, Columbia, and the Massachusetts Institute of Technology, it is not yet peer-reviewed or publicly available but was presented in January.) As Mark Whitehouse of Bloomberg View wrote last month, “They found that proximity to fracking increased the likelihood of low birth weight by more than half, from about 5.6 percent to more than 9 percent. The chances of a low Apgar score, a summary measure of the health of newborn children, roughly doubled, to more than 5 percent.”

Although fracking has frequently been linked to water contamination, Whitehouse notes that drinking chemicals does not seem to pose the greatest risk during pregnancy. “The researchers found similar results for mothers who had access to regularly monitored public water systems and mothers who relied on the kind of private wells that fracking is most likely to affect,” he writes. “Another possibility is that infants are being harmed by air pollution associated with fracking activity.” Miriam Rotkin-Ellman, a public health scientist at the Natural Resources Defense Council, points out that prior studies have linked the ambient presence of chemicals released during natural gas extraction, such as sulfur dioxide, nitrogen dioxide, and benzene, to birth defects.