Marijuana use during pregnancy is associated with a significantly higher risk of the child developing autism, according to a study of more than half a million births.

The researchers behind the study, published in Nature Medicine, found four in 1,000 children born to mothers who reported using cannabis during their pregnancy exhibited signs of autism. For women who did not report using cannabis during their pregnancy, the incidence of autism drops to 2.42 per 1,000 children. This would suggest cannabis use during pregnancy increases the risk of childhood autism by close to 50 percent.

That said, the researchers urge a ‘cautious interpretation’ of the results and stress much more research is required. Not least since the analysis is based on birth records rather than a controlled patient study.

“This is still a database study and it’s not going to answer all the questions,” said the study’s lead investigator Daniel Corsi, a senior research associate at the Ottawa Hospital Research Institute in Canada. “We don’t have perfect data.”

Corsi, whose team in an earlier study suggested marijuana use during pregnancy could lead to premature births, nonetheless advised abstaining from cannabis while pregnant.

“There is an important parallel with alcohol use. 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.

To arrive at their results, the research team analyzed birth data in Ontario taken from 2007 to 2012. Of the approximately 500,000 women who gave birth in that period, around 3,000 self-reported cannabis use whilst pregnant. The researchers then tracked each child’s development up until 2017. In percentage terms, they found 2.2 percent of children whose mothers used marijuana during pregnancy developed autism, while only 1.4 percent of the children whose mothers did not use marijuana during pregnancy developed autism.

To minimize the impact of other factors that could account for higher incidences of autism, the researchers matched pregnant mothers who use cannabis with pregnant mothers who don’t use cannabis along the lines of age, education, health and socioeconomic status, as well as those mothers who’d experienced pregnancy complications.

The study did not, however, consider the impact of varying doses of cannabis, how frequently it is used or whether the use changes over the course of the pregnancy. The results also do not preclude the possibility that women who used marijuana while pregnant may be self-medicating their mental health issues, and that this genetic susceptibility is what is passed on rather than marijuana having an adverse effect.

“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?” said Dr Sven Sandin, a Swedish statistician and epidemiologist who was not involved in the study.

Keely Cheslack-Postava, a research scientist at the New York State Psychiatric Institute, agrees the findings warrant further examination especially since, as the researchers behind the study note, marijuana use during pregnancy has increased in recent years.

“It’s a great use of the data that was there, but I would like to see that kind of evidence in the future to help us really assess if this is a true association,” she said.

Daniel Corsi said in-patient research studies on this question are in the pipeline, potentially to include blood and urine samples to determine the relationship between THC levels during pregnancy and a child developing autism.

While marijuana use during pregnancy may increase the likelihood of the child developing autism, recent research suggests cannabis can be an effective treatment for autistic children with symptoms such as seizures and irritability. Promising tentative results in this regard led to an ongoing large-scale clinical study backed by the US Department of Defense and GW Pharmaceuticals examining the effect of a cannabis compound on irritability and repetitive behaviors in children with autism