Children born to mothers who report using cannabis during pregnancy have about a 50% greater risk of developing autism, research suggests.

While the team behind the work said more research was needed to unpick whether cannabis itself was behind the link, they said the results were concerning.

“There is an important parallel with alcohol use,” said Dr Daniel Corsi, an epidemiologist at Ottawa Hospital Research Institute, and a co-author of the study. “Now the universal recommendation is no alcohol use in pregnancy and I think a similar recommendation should be made for no cannabis use in pregnancy,” he said, adding that it was particularly important given recreational cannabis use was legalised in Canada in 2018.

Writing in the journal Nature Medicine, the team analysed data from around half a million live births in Ontario between 2007 and 2012. The children were followed up until 2017, with autism diagnoses recorded from 18 months. In total about 3,000 of the mothers reported using cannabis during pregnancy.

The results reveal that 2.2% of children born to mothers who used cannabis during pregnancy developed autism, compared with 1.4% of those born to mothers who did not.

To account for other factors that might explain the findings, the team matched 2,364 mothers who used cannabis to 170,671 who did not but who had similar characteristics such as age, education, health conditions and socioeconomic status. They also used modelling to look at additional factors including pregnancy complications.

The results suggest children born to mothers who used cannabis during pregnancy had a 51% greater risk of developing autism than those born to mothers who did not.

However, the study was based on self-reported cannabis use, meaning occasional use was unlikely to be captured. It was also unable to look at the impact of different doses or frequency of cannabis use – or whether cannabis use changed over pregnancy.

Dr Sven Sandin, a statistician and epidemiologist at the Karolinska Institute in Sweden, who focuses on autism and who was not involved in the research, said the study was well conducted and took into account many factors that could explain the apparent link between cannabis use by mothers and autism in their offspring – including age of the mother and rates of pre-term births. But he said autism was relatively rare and the increased risk was small.

Furthermore, he noted that women who reported cannabis use had a higher risk of mental health problems, meaning that they might be self-medicating with cannabis but passing on genetic risk factors for autism to their children.

“We know autism is highly heritable. Could it therefore be that they transfer the risk to their children not through [using cannabis] … but just through passing on their genes?” he said, noting that the possibility was not fully ruled out by the study.

Sir Robin Murray, a professor of psychiatric research at King’s College London, said: “The finding of increased rates of autism in the offspring of mothers who used cannabis in pregnancy is not surprising given the evidence from animal studies showing how it can disrupt brain development.”

Murray noted women who used cannabis when pregnant might also have engaged in unhealthy behaviours that could be behind the apparent link – meaning caution, and further studies, were needed.

“However, this is a useful warning given that many cannabis dispensaries in North America actively promote taking cannabis for morning sickness in pregnancy,” he said.

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