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  • Women who use marijuana during pregnancy are one and half times more likely to have a child with autism than those who don't.
  • The study, involving more than a half million women, follows past research suggesting pot use is linked to lower birth weight.
  • While the study had flaws like being unable to show cause-and-effect, the authors say it suggests pot use should be recommended against during pregnancy, just like alcohol is.
  • Visit Business Insider's homepage for more stories.


Using marijuana during pregnancy is linked to 50% greater chance of having a child with autism, according to the largest study of its kind.

The study, published in Nature Medicine on Monday, reviewed data from more than a half a million women in Ontario, Canada - about 3,000 of whom reported using cannabis during pregnancy and about 2,200 of whom reported using cannabis and no other substances.

They found that 2.2% of women who used marijuana had children with autism compared to 1.4% of women who did not use cannabis but had similar characteristics, like age, education, and socio-economic status.

The study authors say that while their study was imperfect, the results are worrying, especially given that cannabis was illegal in Canada during the period (2007 to 2012) the data was collected.

"The universal recommendation is no alcohol use in pregnancy and I think a similar recommendation should be made for no cannabis use in pregnancy," co-author Dr. Daniel Corsi, an epidemiologist at The Ottawa Hospital and BORN Ontario, told the Guardian.

The research is growing that marijuana use during pregnancy can have lasting effects on children

The findings follow the same team's research showing marijuana use during pregnancy increases the risk of other complications like preterm birth.

One study from 2018 in Colorado found that women who reported using cannabis while pregnant had a 50% chance of lower birth weights.

Other research has shown that marijuana use during pregnancy may affect a child's brain development. It appears to be linked to lower IQs, attention problems, and more impulsiveness.

Such research led to the US Food and Drug Administration issuing a statement in 2019 about the dangers of using CBD and THC, two of the main compounds found in cannabis, while pregnant or breastfeeding, Insider's Julia Naftulin previously reported.

But still, cannabis use during pregnancy appears to be on the rise, perhaps as a way to cope with nausea. According to a January 2019 letter in JAMA Pediatrics, marijuana use during pregnancy in the U.S. increased from 2.9% in 2002 to 5% in 2016.

The study couldn't show cause and effect

The study didn't identify how much cannabis the women were using, how often, when, or the method used. Self-reported data is faulty, too.

The research also only showed a link between marijuana use and autism, but couldn't prove cause and effect. For instance, there could be something that drives certain women to use pot during pregnancy that also increases their risk of having children with autism.

But while more research is needed, the study should inform discussions doctors have with parents-to-be, the study authors said.

"In the past, we haven't had good data on the effect of cannabis on pregnancies," Corsi said in a press release. "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."