Based on a decade of data from Pennsylvania, researchers report that babies born to mothers living within 1 kilometer of active "fracking" wells are 25% more likely to exhibit low birthweight - a risk factor for infant mortality, ADHD, asthma, and other negative outcomes. The results reflect a possible health consequence of exposure to fracking pollutants. To date, concerns about the impact of the fracking process on human health, particularly at the local level, are unresolved. To further evaluate such impacts, Janet Currie and colleagues analyzed records of more than 1.1 million births in Pennsylvania from 2004 to 2013. They compared the birthweights of babies born to mothers living within 1, 2 or 3 kilometers of fracking wells - both before and after the wells were active. To be more certain in identification of health influences specifically related to well proximity, Currie et al. evaluated birthweights of siblings born at different distances to wells, both close enough to be exposed to fracking in utero, and not. While there is some evidence that living within 3 kilometers of an active fracking site can create negative health effects in utero, the researchers report, the largest health impacts are to infants born to mothers living within 1 kilometer of active wells; these individuals are 25% more likely to be classified as low birthweight, the authors say.