Pregnant women who live close to natural gas wells utilizing hydraulic fracking are more likely to deliver babies with low birth weights, an analysis of birth records suggests.

That's the word from researchers at the University of Pittsburgh who conducted an analysis of birth records in southwestern Pennsylvania.

While the result doesn't prove a direct cause-and-effect connection between proximity to a high density of fracking wells and lower birth weights, it suggests a worrisome association that merits further investigation, they say in their study published in PLOS ONE.

"These findings cannot be ignored," says study co-author Bruce Pitt of the university's Department of Environmental and Occupational Health. "Our work is a first for our region and supports previous research linking unconventional gas development and adverse health outcomes."

Unconventional development can include horizontal drilling as well as hydraulic fracturing, commonly called "fracking," which allows recovery of significant amounts of natural gas tightly bound in shale deposits.

Before 2007, such technologies were used in just 44 wells drilled in the Marcellus Shale region of Pennsylvania; between 2007 and 2010, that number grew to more than 2,800 wells, the researchers report.

They looked at birth outcomes for 15,451 babies delivered in Butler, Westmoreland and Washington counties and cross-reference that with the nearness of the mothers' homes to wells using fracking or other unconventional forms of gas development.

Mothers living in homes closest to a high density of those wells were 34 percent more likely to deliver babies considered "small for gestational age" than was seen with mothers whose homes were farther away, the researchers report.

After adjusting for other factors that can affect newborn weight — level of prenatal care, mothers who smoked, age, education and race — the finding of lower birth weight still held, they say.

"Developing fetuses are particularly sensitive to the effects of environmental pollutants," Pitt says. "We know that fine particulate air pollution, exposure to heavy metals and benzene, and maternal stress all are associated with lower birth weight."

Benzene is sometimes found in what are known as "flowback" waste fluids produced by fracking, and burning of natural gas at well heads can release volatile organic compounds.

Pitt emphasizes that his study cannot definitely link pollutants with lower birth weight babies and that much more research is needed, ideally with a greater number of pregnant women, with an assessment of their exposure to both fracking gas wells and other possible pollutants.

Still, he says, this small initial study may be cause for concern.

"There is a clear need for studies in larger populations with better estimates of exposure and more in-depth medical records," he says.

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