Pregnant women in rural California who lived near active oil and gas wells were 40 percent more likely to give birth to low birth-weight babies, according to new research published today.

The study led by University of California scientists is the first to investigate what California’s constellation of oil and gas development means for babies born nearby. The finding could galvanize efforts in the state Legislature to require buffer zones around oil and gas activities.

The researchers found that 6 percent of women living near rural oil and gas wells that churned out more than 100 barrels a day had low birth-weight newborns, compared to 5 percent of women with no oil and gas production nearby. When the researchers factored in variables like the mother’s age and socioeconomic status, that translates to a 40 percent increased likelihood.

Low birthweight babies, who weigh less than 5.5 pounds at birth, may be healthy but often have a higher rate of illnesses, such as respiratory diseases and difficulty fighting infections, as well as developmental delays.

The researchers reviewed nearly 3 million birth certificates from 2006 to 2015 in the Sacramento Valley, San Joaquin Valley, South Central Coast and Los Angeles Basin.

While the link between oil and gas production and low birth-weight babies was found in rural areas, it didn’t hold up in urban areas, such as large parts of the Los Angeles region.

But a well churning out oil in a city backyard is not necessarily benign.

“I don’t think it means that urban people don’t have to worry,” said study senior author Rachel Morello-Frosch, a professor of public health and environmental science, policy and management at UC Berkeley.

There was a small increase in the odds of babies born small for their gestational age for women who lived near urban areas with oil production hot spots. Plus, Morello-Frosch said it’s possible the link to low birth-weight babies is there, but it’s just hard to spot because oil and gas might produce a smaller share of the overall pollution in urban areas. In addition, people in rural regions are exposed to pollutants — in groundwater, for instance — that might make them more vulnerable to pollution from oil and gas production.

The findings, published in the journal Environmental Health Perspectives, support a handful of studies in other states, including Colorado and Pennsylvania. Those earlier studies reported increased odds of health effects among babies born near oil and gas development, including premature births, heart defects, and low birthweight.

No one had looked at the effects on babies in California, with its old infrastructure and unique mix of methods for drawing oil and gas out of the ground. Only about 20 percent of California’s oil comes from hydraulic fracturing, a process that uses high pressure fluids to fracture bedrock, and sand to prop open the cracks. Fracking has led to more oil and gas production across the U.S. and increased concerns about pollution and groundwater contamination.

“There’s been a lot of attention on hydraulic fracturing. Many from the industry and from the scientific community have thought that a lot of the exposure [to pollutants] was in the drilling process,” said Elaine Hill, an assistant professor of public health sciences at the University of Rochester who was not involved in the study.

Hill said the new findings suggest it’s not just about drilling. “The takeaway is active wells may continuously expose people living nearby, even after drilling the well is finished.”

Other factors could be at play, too

The study doesn’t address what factors related to oil and gas development might lead to adverse birth outcomes. But many hazardous air and water pollutants are linked to drilling and oil production, such as fine airborne particles, mercury and volatile organic compounds like benzene. There’s also commercial activity associated with energy development, which brings truck traffic, dust, noise and light.

Sabrina Demayo Lockhart, a spokesperson for the California Independent Petroleum Association, said socioeconomic factors could explain the findings, such as access to prenatal care, low incomes and underlying health conditions.

“Pinpointing direct health outcomes to one highly regulated activity ignores the fact that there are so many socioeconomic variables that can impact public health,” she said. “California has the strongest environmental protections for oil and natural gas activity. Extremists will use the headlines to generate fear in their push for stricter regulations.”

The research comes as California weighs the future of oil exploration in the state. In November, California’s oil and gas regulator, the Geologic Energy Management Division, or CalGEM, pledged to strengthen oversight of hydraulic fracturing and banned new oil wells using extraction methods tied to leaks in Kern County. But in April, the state resumed approving hydraulic fracturing permits, ending a nine-month hiatus.

A bill by Assemblyman Al Muratsuchi, a Democrat from Torrance, to require a minimum buffer around new oil and gas development stalled in the Legislature last year. It now awaits a decision in the state Senate.

“Big Oil continues to be one of the biggest special interest lobbies in Sacramento,” Muratsuchi said. “We’ve always known it was going to be a tough fight, but we’re ready for the tough fight, and we’ll keep fighting for this.”

California cultivates a climate-forward image, but also is the seventh largest producer of crude oil in the country. Oil wells can be spotted near the Beverly Hills High School campus, in residential areas of Long Beach and along Kern County roads.

The Environmental Defense Fund estimates that more than 890,000 Californians live within half a mile of oil and gas wells in the state. And Morello-Frosch wanted to know what that proximity meant for the babies born nearby. Her team dug through birth certificates for babies born in places with heavy oil and gas development, looking for infants whose mothers lived within about 6.2 miles of oil or gas wells.

“We realized that no one had actually looked at the potential health effects of oil and gas development activities in the state of California,” Morello-Frosch said. “Despite the fact that California is still a pretty significant producer of oil and gas.”

The researchers found that mothers in rural areas who lived within 1 kilometer — 3,300 feet — of wells producing more than 100 barrels of oil or the natural gas equivalent per day had the highest odds for adverse outcomes in their babies.

There was no increase in the likelihood of preterm births. But in addition to the increase in low birth-weight babies, mothers in rural oil and gas hot spots were 22 percent more likely to give birth to babies that were small for their gestational age. Even babies born at term were about 1.3 ounces lighter, on average.

From an individual’s health perspective, Morello-Frosch said 1.3 ounces may not seem like a big deal — but a downward shift in birth weight at the population level could be concerning.

“All of a sudden the proportion of low birth weight babies you have just got bigger,” she said. “And the proportion of low birthweight babies that might have developmental problems as they get older has increased.”

Rupa Basu, an epidemiologist with California’s Office of Environmental Health Hazard Assessment who did not participate in the study, wishes that the study delved into what exactly the women were exposed to near oil and gas wells.

“There’s people living near them,” Basu said. And people want to know, “Is it safe for me to live this close? And the answer often is we don’t know yet.”

Still, Basu said, it’s an important issue to explore. “This is obviously a big concern — being that it’s pregnant women and their offspring.” is a nonprofit, nonpartisan media venture explaining California policies and politics.