Women who are exposed to high levels of air pollution during their third trimester of pregnancy may be twice as likely to have an autistic child, a study found.

Researchers from the Harvard School of Public Health found the risk of autism rises in parallel with exposure to fine particulate matter during pregnancy, with the biggest effect occurring in the final months. The results appear in Thursday’s edition of Environmental Health Perspectives.

The findings add to other research suggesting the environment plays a role in autism, a developmental disorder marked by repetitive behaviors and trouble and socializing. The study, which started in 1989 and involved more than 100,000 nurses from across the U.S., will help researchers home in on the causes of autism and potential ways to prevent it, said researcher Marc Weisskopf.

“One of the unique aspects of the study we did is that it provides an even stronger piece of evidence for there being a causal effect,” said Weisskopf, an associate professor of environmental and occupational epidemiology at Harvard. “It’s really the pollution doing it.”

Autism, thought to affect 1 in 68 children in the U.S., is typically diagnosed after behavioral changes start to develop before the age of 5. Studies suggest it may begin when certain brain cells fail to mature within the womb. Researchers focused on 1,767 children born from 1990 to 2002, including 245 diagnosed with autism. The design of the study and the results rule out many confounding measures that can create a bias, Weisskopf said. The researchers took into account socioeconomic factors that can influence exposure to pollution or play a role in whether a child is diagnosed with autism. The fact that pollution caused problems only during pregnancy strengthened the findings, he said.

Other researchers warned, however, that the study is far from definitive.