Humans have lived side by side with oil development for over a century, but it’s only recently that scientists started tracking the health consequences of that proximity. A new paper published today in the journal Environmental Health Perspectives shows that pregnant women in rural areas who lived within one kilometer of high-producing wells were 40% more likely to have low birth weight babies compared to those who lived further away. Birth weights below 5 lbs. 8 oz. correlate with higher risk of health problems in early childhood, sometimes even carrying into adulthood.

Rachel Morello-Frosch, an environmental health and justice researcher at the University of California at Berkeley and the lead author of the study, said that even though California is a top oil and gas state, hers was the first broad-based study to tackle related health impacts in the state.

Her team looked at the records of nearly 3 million births to women living within 10 kilometers (6.2 miles) of at least one oil or gas well, active or inactive, between 2006 and 2015. In rural areas near the most active wells, they found, women were 20% more likely to give birth to babies relatively small for their gestational age, even if they didn’t reach the threshold for low birth weight. Babies carried to full term were 1.3 oz. smaller on average than those born to mothers who don’t live near wells. The researchers found no evidence of higher mortality for the babies born near wells.