Pregnant women in Pennsylvania who live near a high density of natural gas wells had a greater chance of having babies with low birth weights.

Pregnant women who live near hydraulic fracturing sites are giving birth to babies with lower birth weights, a new study contends.

Mothers whose homes are near a high density of so-called “fracking” gas wells are as much as 34 percent more likely to have babies who are “small for [their] gestational age,” according to researchers from the University of Pittsburgh Graduate School of Public Health.

Fracking, a shortened term for hydraulic fracturing, allows oil and gas companies to access the natural gas trapped in shale deposits.

Southwestern Pennsylvania, the area the researchers studied, is home to the Marcellus Shale deposit. Before 2007, it was home to just 44 known wells using hydraulic fracturing. Now, there are at least 2,864 such wells.

Researchers divided birth data into four groups depending on the number and proximity of wells within a 10-mile radius of the mother’s home. They looked at 15,451 births that occurred between 2007 and 2010 in Pennsylvania’s Washington, Westmoreland, and Butler counties.

Mothers whose homes were closest to more wells were 34 percent more likely to have low birth weight babies than mothers whose homes were in the bottom quarter of well proximity. Babies categorized as having a low birth weight fell within the smallest 10 percent of babies born at that point in pregnancy.

The researchers took into account other factors that could influence a newborn’s weight, including whether the mother smoked, her prenatal care, race, education, age, and whether she’d had previous babies. They said their findings remained consistent.

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Pollutants associated with fracking could cause the low birth weights.

“Developing fetuses are particularly sensitive to the effects of environmental pollutants,” said study co-author Bruce Pitt, Ph.D., chairman of the university’s Department of Environmental and Occupational Health. “We know that fine particulate air pollution, exposure to heavy metals and benzene, and maternal stress all are associated with lower birth weight.”

In southwestern Pennsylvania the waste fluids produced through hydrofracturing can contain benzene, the researchers said. Unconventional gas development also creates an opportunity for air pollution through flaring of methane gas at the well heads and controlled burning of natural gas that releases volatile organic compounds, including benzene, toluene, ethylbenzene, and xylene.

But the data, published today in the journal PLoS ONE, do not prove that proximity to the wells caused the lower birth weights. They merely show a correlation.

“These findings cannot be ignored,” said Pitt. But “there is a clear need for studies in larger populations with better estimates of exposure and more in-depth medical records.”

Read More: Fracking Contains at Least 80 Highly Toxic Chemicals »

Katie Brown, spokesperson for Energy in Depth, a program of the Independent Petroleum Association of America, said the study is contradictory because the lowest birth weights in the region were recorded in the neighborhoods farthest from the wells.

Brown added hydraulic fracturing is usually done near poorer and rural areas, which historically have more health-related problems.

“Opponents of drilling have argued that shale development is happening in economically depressed areas, but it’s in those same areas where we often see higher incidences of public health issues, irrespective of shale gas development,” she said.

The oil drilling produces more benefits than problems in these regions, Brown said.

“If anything, shale development has given many rural regions economic hope, boosting well-being by providing residents with jobs and better access to healthcare,” she said.

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