Dear Doctor:

Fracking just started near my Ohio hometown, and I remember reading about a connection to the risk of having a baby with low birth weight. I want to get pregnant, but now I’m worried. How close do you have to be to a site to be affected?

Dear Reader: “Fracking” is the common term for hydraulic fracturing, a drilling process that pumps fluid at ultra-high pressure deep into the earth. This fractures the underlying shale rock and makes the natural gas or oil that it contains available for extraction. There has been no end of controversy regarding the process in recent years, with vigorous debate over whether or not fracking causes air pollution, contaminates surface and groundwater, leads to earthquakes and plays a role in a range of health problems. Among the health questions that have been raised is whether the process affects birth weight among pregnant women who live near fracking sites.

Your question refers to a study published a few years ago in the journal Science Advances. The researchers, including one from UCLA, analyzed the outcomes of 1.1 million live births in Pennsylvania between 2004 and 2013. They compared the birth weights of infants born to mothers living at varying distances from active fracking sites, both before the extraction operations had begun, and after the wells became active. They found that women who lived very close to an active fracking operation — within one-half mile — had a 25 percent higher risk of delivering a baby of low birth weight than did women who lived at a distance of 2 miles or more.

The study found that babies born to women who lived more than one-half mile from a fracking site, but less than 2 miles away, were also adversely affected, but to a lesser degree. When researchers looked at women who lived at least 2 miles from a fracking operation, they found no signs of adverse health effects to newborns.

“Low birth weight” is the term used to describe an infant who weighs less than 5 pounds, 8 ounces at delivery. By contrast, the average healthy newborn weighs in at about 8 pounds. Low birth weight occurs most often in premature births, when the infant doesn’t have the full 37 weeks of gestation to grow and gain weight. Low birth weight is sometimes seen in full-term babies who failed to grow well during gestation due to issues with the mother’s health, the placenta or the baby’s condition. Multiple births also often result in smaller babies.

The reason low birth weight is a concern is that smaller babies are at greater risk of complications after delivery, including low oxygen levels, difficulty feeding, gastrointestinal problems and SIDS, or sudden infant death syndrome. Low birth weight has also been shown to be a risk factor for future health and developmental problems such as asthma, Type 2 diabetes, attention deficit disorder, learning disabilities and hyperactivity.

Although the study correlates proximity to fracking sites with low birth weight, it doesn’t explain what is causing these outcomes. The authors agree that more information is needed, and have said they hope their study will lead to further research.