Women who smoke cannabis during pregnancy almost double the risk of their child having autism, a major study of half a million births has found.
The research, published in the journal Nature Medicine, revealed the incidence of autism was 4 cases for 1000 people observed in one year who were exposed to cannabis in pregnancy, compared with 2.42 among unexposed children.
The researchers reviewed data from every birth in Ontario between 2007 and 2012, before recreational cannabis was legalised in Canada.
The children were followed up until 2017 and autism diagnosis was recorded from 18 months.
Of the half a million women in the study, around 3,000 (0.6 per cent) reported using cannabis during pregnancy.
The team had previously found that cannabis use in pregnancy was linked to an increased risk of premature birth.
In that study, they found that women who used cannabis during pregnancy often used other substances including tobacco, alcohol and opioids.
Considering those findings, in the current study the researchers specifically looked at 2,200 women who reported using only cannabis during pregnancy, and no other substances.
The findings showed that babies born to this group still had an increased risk of autism compared to those who didn't use cannabis.
“Children with prenatal cannabis exposure had an increase of 50 per cent in the risk of an autism diagnosis over the study period,” the authors said, even when controlling for outside factors.
But the researchers noted the self-reporting method of the study did result in limitations, as they do not know how much cannabis the women were using, how often, at what time during their pregnancy, or how it was consumed.
“Cannabis users may be misclassified as nonusers in the birth registry, probably due to underreporting, which may arise from social stigma or fear of involvement of child protective services,” the authors said.
Dr Daniel Corsi, an epidemiologist at The Ottawa Hospital, said this was one of the “largest studies on this topic to date” and previously they did not have “good data” of the effect of cannabis on pregnancies.
"We hope our findings will help women and their health-care providers make informed decisions,” he said.
Professor Mark Walker, of the University of Ottawa in Canada, said: "There is evidence that more people are using cannabis during pregnancy.
"This is concerning, because we know so little about how cannabis affects pregnant women and their babies.
"Parents-to-be should inform themselves of the possible risks, and we hope studies like ours can help."