Children of mothers who use cannabis during pregnancy appear to be at higher risk of being autistic, according to a study of half a million women.
In the largest study of its kind, researchers found four in 1000 children whose mothers used cannabis during pregnancy showed incidence of autism, compared to 2.42 among those whose mothers didn’t use it.
The study, conducted at the University of Ottawa, reviewed data between 2007 and 2012, before recreational cannabis use was legalised in the area.
There were 500,000 women in the study, around 3,000 of whom reported using cannabis during pregnancy.
The paper also reports “the incidence of intellectual disability and learning disorders was higher among offspring of mothers who use cannabis in pregnancy” although it says this finding is less statistically robust.
Canada has legalised cannabis use but recommend not to consume it if you are pregnant or breastfeeding.
"Despite these warnings, there is evidence that more people are using cannabis during pregnancy," said Dr Mark Walker, Chief of the Department of Obstetrics, Gynecology and Newborn Care at The Ottawa Hospital, professor at the University of Ottawa and senior author on the study.
"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."
In the study, published in Nature Medicine, the researchers looked at results from 2,200 women who said they had used cannabis and no other substance. However the study did not take into account how much cannabis was used, how often it was used, at what time in the pregnancy it was used or how it was consumed.
Therefore the study can only show association, not cause and effect.
“In the past, we haven't had good data on the effect of cannabis on pregnancies,” said Dr Daniel Corsi, epidemiologist at The Ottawa Hospital.
“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.”