Pregnant women living near hydraulic fracturing wells are much more likely to give birth prematurely or have high-risk pregnancies, according to a new study from the Johns Hopkins Bloomberg School of Public Health.

Hydraulic fracturing, more commonly known as fracking, is the process of drilling into the earth and pumping chemicals, water and sand into underground shale rock to extract embedded oil or natural gas.

Researchers found that pregnant women living near active fracking sites were 40 percent more likely to give birth before 37 weeks of gestation, which is considered preterm. They also found a 30 precent increase in pregnancies labeled "high-risk."

Both are associated with a range of medical problems for the baby, including breathing problems, cerebral palsy, hearing and vision impairments, developmental delay and long-term neurological disabilities, according to the Centers for Disease Control and Prevention. "Preterm-related causes of death together accounted for 35% of all infant deaths in 2010, more than any other single cause," the CDC said, adding that preterm births also cost the U.S. health care system more than $26 billion in 2005.

The Johns Hopkins team was unable to pinpoint the exact reason for the fracking risk, but lead researcher Brian Schwartz said that every step in the drilling process has an environmental impact.

"Now that we know this is happening we'd like to figure out why," said Schwartz, a professor at the Johns Hopkins Bloomberg School of Public Health. "Is it air quality? Is it the stress? They're the two leading candidates in our minds at this point."

Schwartz and his team looked at medical records of 9,384 women who gave birth in counties in northern and central Pennsylvania and compared the data with information on natural gas wells to determine how close the women lived to fracking wells.

"The first few studies have all shown health impacts," Schwartz says. "Policymakers need to consider findings like these in thinking about how they allow this industry to go forward."

The past decade has seen a fracking boom across the U.S., especially in states like Pennsylvania, Texas, Colorado, North Dakota and Arkansas.

"More than 8,000 unconventional gas wells have been drilled in Pennsylvania alone," Schwartz said, reported Think Progress. "We're allowing this while knowing almost nothing about what it can do to health. Our research adds evidence to the very few studies that have been done in showing adverse health outcomes associated with the fracking industry."

Other studies have linked fracking to low birth weight in babies and respiratory and skin conditions in the general population living near natural gas wells. Researchers in Texas also found higher levels of carcinogenic chemicals in drinking water near fracking sites.