A large sample of women in Canada who used marijuana during pregnancy were 1 1/2 times as likely to have a child with autism as those who didn't, a new study has found.
The retrospective analysis, involving more than half a million women, follows past research suggesting pot use is linked to lower birth weight.
While the study does not prove cause and effect, the authors say it suggests pot use should be recommended against during pregnancy in the way alcohol is.
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Marijuana use during pregnancy has been linked to a 50% greater chance of having a child with autism, according to the largest study of its kind.
The retrospective analysis, published in Nature Medicine on Monday, reviewed data from more than half a million women in the Canadian province of Ontario — about 3,000 of whom reported using cannabis during pregnancy and about 2,200 of whom reported using cannabis and no other substances.
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The researchers found that 2.2% of women who used marijuana had children with autism, compared with 1.4% of women who did not use cannabis but had similar characteristics, like age, education, and socioeconomic 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) when 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," the coauthor Dr. Daniel Corsi, an epidemiologist at The Ottawa Hospital and BORN Ontario, told The Guardian.
More and more evidence suggests marijuana use during pregnancy can have lasting effects on children
The findings follow the same team's research linking marijuana use during pregnancy to an increased 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 the US Food and Drug Administration to issue a statement in 2019 warning against 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. A January 2019 letter in JAMA Pediatrics estimated that marijuana use during pregnancy in the US increased from 2.9% in 2002 to 5% in 2016.
The study couldn't show cause and effect
While finding a link between marijuana use and autism, the study 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.
The study also didn't identify how much cannabis the women were using, how often, when, or the method used. Self-reported data can be faulty, too.
But while more research is needed, the study authors said their work should inform discussions doctors have with parents-to-be.
"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 healthcare providers make informed decisions."
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