PITTSBURGH — Researchers have found an association between lower birth weights for babies and their mothers’ proximity to unconventional gas wells in a study that included birth records from Butler County.

The findings, published by scientists at the University of Pittsburgh, included more than 15,000 total birth records from 2007 to 2010 and 4,733 births from Butler County, said a university spokesman.

The study included birth and gas well records from Butler, Westmoreland and Washington counties.

The study found that mothers whose homes were closest to areas with a higher density of unconventional wells were 34 percent more likely to have babies who were “small for gestational age” than mothers whose homes were in areas less dense with unconventional wells.

In their conclusions, researchers call the findings a “small but significant association” that held up even after accounting for factors such as smoking, access to prenatal care and the mothers’ age.

Small gestational age refers to babies whose birth weight ranks them below the smallest 10 percent compared to other births.

Researchers said they found no evidence linking premature births to the density or proximity of wells to mothers’ homes.

The scientists from Pitt’s School of Public Health, Magee-Women’s Research Institute and the Pitt School of Medicine also said their findings don’t amount to causation and are limited by the kind of data used.

The researchers used online birth records of the Pennsylvania Department of Health to get data for their study.

“I think the medical questions that are being posed have to be broadened beyond what can be obtained on birth certificates,” said Dr. Bruce Pitt, a co-author of the study.

Pitt said the work, which appeared Wednesday in the scientific journal Plos One, highlights the need for larger, more complex studies on the possible health impacts of unconventional gas drilling, also known as hydraulic fracturing.

“This is a complicated issue,” he said. “There needs to be some cooperation between industry, government, scientists and the people who do public policy to move forward in a rational way.”

Pitt said gathering the necessary data for such a study could take years, given the kinds of medical outcomes researchers want to explore.

The study provoked a lengthy online response from the Marcellus Shale Coalition.

In a blog post, the Pittsburgh-based industry advocacy group said the university’s work was skewed by a variety of factors and used “thoroughly debunked ‘research’ as its basis.”

The group said researchers failed to provide “critical context” for the term “low birth weight.”

“I think there’s a lot of shortcomings here,” said Travis Windle, managing director of strategic communications for FTI Consulting for the coalition.

The coalition pointed to a separate study published Thursday by the U.S. Environmental Protection Agency, which found that groundwater pollution from hydraulic fracturing occurred in “specific instances” but was not widespread.

Coalition President Dave Spigelmyer said the EPA study “reflect(s) the strong, rigorous and modernized regulations in place that ensure environmental protection.”

A university spokesman said researchers found the highest density of wells in Butler County was clustered just east of Harmony.

According to online records kept by the state Department of Environmental Protection, 68 unconventional gas wells were drilled in Butler County from 2007 to 2010.