The enormous growth of unconventional natural gas fracturing (also known as fracking) in recent years has come at the expense of knowing little if anything about the health risks associated with this practice. As production as slowed due to dropping gas prices in the past year or so, several studies have come out that raise serious questions about the health impact of this process. A study published earlier this month by a group of researchers at the John Hopkin’s School of Public Health concluded that “expectant mothers who live near active natural gas wells operated by the fracking industry in Pennsylvania are at an increased risk of giving birth prematurely and for having high risk pregnancies.” This paper was published in the journal Epidemiology.

In this paper, the authors examined more than 9,000 births in 40 counties in northern and central Pennsylvania between January 2009 and January 2013. They compared electronic birth outcome data with information that estimated the cumulative exposure to fracking activity in the region. This information included how close wells were to homes where the mothers lived, what stage of drilling the wells were in, the depth of the wells, and how much gas was generated from the well during the mother’s pregnancy. This information was used to generate a cumulative index of how active each of the wells were and how close they were to the women.

They found that living in the most active area of drilling and production activity was associated with a 40 percent increase in the likelihood of a woman giving birth before 37 weeks of gestation (considered pre-term) and a 30 percent increase in “high risk” pregnancies, a designation that can include elevated blood pressure and excessive weight gain during pregnancy. In total, 11 % of the pregnancies were born preterm, with 79% born between 32 and 36 weeks.

Other research in recent years has also shown a connection between fracking wells and low birth weight. “There are now four studies that have looked at various aspects of reproductive health in relation to this industry and all have found something,” Brian Schwartz, the lead author of the Hopkins study, said in an interview. In one of these studies, researchers found an increased risk of congenital heart and neural tube defects in babies whose mothers lived within 10 miles of a natural gas well in rural Colorado.

In a media statement released with the study, the authors made clear that the study can’t pinpoint the specific reason why pregnant women living near the most active wells had the worst pregnancy outcomes. But Schwartz pointed out that every step of the drilling process has an environmental impact. “When the well pads are created, diesel equipment is used to clear acres of land, transport equipment and drill the wells themselves. Drilling down thousands of feet and then horizontally many more thousands of feet requires heavy equipment to break up the shale where the gas sits. Hydraulic fracturing (fracking) then involves injecting millions of liters of water mixed with chemicals and sand to fracture the shale. The fluids are then pumped back to the surface. The gas itself also releases pollutants.” Schwartz also noted that living near fracking well results in increased noise, road traffic and other changes that can increase maternal stress levels.

“Now that we know this is happening we’d like to figure out why,” Schwartz says. “Is it air quality? Is it the stress? They’re the two leading candidates in our minds at this point.”

As with many other environmental and public health risks, the more we look, the more we find. We already know that fracking contributes to the impact of climate change because of the large amount of methane that’s released. It’s beginning to look more and more like it also has serious effects on the health of the people who live nearby.