Children whose mothers used marijuana in pregnancy may have a higher risk of autism, researchers report in Nature Medicine.

The University of Ottawa researchers analyzed data on every birth in Ontario, Canada between April 2007 and March 2012. They found pregnant women who used marijuana were 1.5 times more likely to have a child with autism compared with women who didn’t use the drug, HealthDay reports.

Of the half a million women in the study, about 3,000 (0.6%) reported using marijuana during pregnancy. The study focused on the 2,200 women who used marijuana but did not use other substances during their pregnancy.

The study data did not include information on the particulars of the women’s marijuana use, such as how much marijuana the women were using, how often, at what time during their pregnancy, or how it was consumed.

The same research team previously found that marijuana use in pregnancy was linked to an increased risk of preterm birth.