Marijuana use during pregnancy linked to greater risk of autism in children, study says

OTTAWA, Ontario - In the largest study of its kind, researchers found that marijuana use during pregnancy was linked to an increased risk of the child developing autism.

“Despite these warnings, there is evidence that more people are using cannabis during pregnancy,” said study author Dr. Mark Walker, a professor at the University of Ottawa and chief of the Department of Obstetrics, Gynecology and Newborn Care at The Ottawa Hospital. “This is concerning, because we know so little about how cannabis affects pregnant women and their babies.”

The study, published Aug. 10 in the journal Nature Medicine, found that women who used cannabis while pregnant were 1.5 times more likely to have a child develop autism.

Researchers reviewed data from births in Ontario, Canada between 2007 and 2012, before recreational marijuana was legalized in the country. Of the 500,000 women reviewed in the study, about 3,000, or 0.6%, reported using cannabis during pregnancy.

In the study, researchers looked at 2,200 of those women who reported using only cannabis and no other substances during pregnancy. They found that babies born to this group had an increased risk of autism compared to those who did not use cannabis.

“The incidence of autism spectrum disorder diagnosis was 4.00 per 1,000 person-years among children with exposure, compared to 2.42 among unexposed children, and the fully adjusted hazard ratio was 1.51,” according to the study.

The researchers did not know how much cannabis the women were using, how frequently, the method of which it was consumed, nor at what point during their pregnancy it was used. While the researchers tried to control for other factors that could influence neurological development, the study can still only show an association between cannabis use and autism risk in children — not cause and effect.

The study was conducted by researchers at the University of Ottawa, The Ottawa Hospital and BORN Ontario, who previously found that cannabis use in pregnancy was linked to an increased risk of preterm birth. In that study, they found that women who used marijuana during pregnancy often used other substances, such as tobacco, alcohol and opioids.

As marijuana use becomes more widely legalized and socially acceptable around the world, more women have turned to it to help with morning sickness, among other pregnancy symptoms. A study published in 2018 found that pregnant women with severe nausea and vomiting in the first trimester were nearly four times more likely to use marijuana during this time than those who didn’t have symptoms.

Researchers say that women who are thinking about or currently using marijuana during pregnancy should talk to their doctor to help make an informed choice about what is best for them and their baby.

“In the past, we haven’t had good data on the effect of cannabis on pregnancies,” said study author Dr. Daniel Corsi, an adjunct professor at Ottawa’s School of Epidemiology and Public Health and an epidemiologist at The Ottawa Hospital and BORN Ontario. “This is one of the largest studies on this topic to date. We hope our findings will help women and their health-care providers make informed decisions.”

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This story was reported from Cincinnati.