In what they call the largest study ever done, researchers found using marijuana while pregnant may increase the risk that a child will develop autism.

“Women who used cannabis during pregnancy were 1.5 times more likely to have a child with autism,” said study author Dr. Darine El-Chaâr, a maternal fetal medicine specialist and clinical investigator at Ottawa Hospital Research Institute in Canada.

“These are not reassuring findings. We highly discourage use of cannabis during pregnancy and breastfeeding,” she said.

Past studies have shown the use of marijuana during pregnancy is linked to low birth weight, impulsivity, hyperactivity, attention issues and other cognitive and behavioral issue in children, according to the US Centers for Disease Control and Prevention. Pregnant women who use marijuana, one study found, have a 2.3 times greater risk of stillbirth.

“Based on that,I’m not too surprised by these findings,” El-Chaâr said. “Fetal brain development occurs throughout all gestational ages.”

The study, published Monday in the journal Nature, reviewed data from every birth in Ontario, Canada, between 2007 and 2012, well before recreational marijuana was legalized in Canada in 2017. Of the half a million women in that data pool, researchers then narrowed the study to 2,200 women who said they used only marijuana during pregnancy, without mixing it with tobacco, alcohol or opioids.

The study did not capture the amount and type of marijuana the women used during pregnancy. Nor did the study know when during the pregnancy or how often women used it. And while the study could only show association, not cause and effect, researchers said they did their best to eliminate confounding factors.

Medical reference: CNN Health