Pregnant women who smoke cannabis almost double the risk of their baby being born autistic, warns a new study.
In the largest ever study of its kind, researchers found that children whose mothers reported using cannabis during pregnancy were at greater risk of autism.
The incidence of autism was four per 1,000 person-years among children exposed to cannabis in pregnancy, compared to 2.42 among unexposed children.
‘There is evidence that more people are using cannabis during pregnancy,’ said senior study author Professor Mark Walker, of the University of Ottawa in Canada.
‘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.’
A Canadian study found that rates of autism were twice as high among the children of women who used marijuana during pregnancy, compared to rates among children of mothers who did not use the drug (file)
The researchers reviewed data from every birth in Ontario between 2007 and 2012, before recreational cannabis was legalised in Canada.
Of the half a million women in the study, about 3,000 (0.6 per cent) reported using cannabis during pregnancy.
Importantly, these women reported using only cannabis.
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 the 2,200 women who reported using only cannabis during pregnancy, and no other substances.
The findings, published in the medical journal Nature Medicine. showed that babies born to this group still had an increased risk of autism compared to those who didn’t use cannabis.
The researchers do not know exactly how much cannabis the women were using, how often, at what time during their pregnancy, or how it was consumed.
But as cannabis becomes more socially acceptable, doctors are concerned that some parents-to-be might think it can be used to treat morning sickness.
Dr Daniel Corsi, an epidemiologist at The Ottawa Hospital, said: ‘In the past, we haven’t had good data on the effect of cannabis on pregnancies.’
He added: ‘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.’
Autism is fairly common, but still poorly understood.
In the US, about one in every 59 children born will fall somewhere on the autism spectrum.
About one in every 66 children in Canada are autistic and, globally, the rate is approximately one in every 160 children.
Research suggests that there is likely some genetic basis for autism, which is about four-times more common among boys than girls.
But scientists believe exposures in the womb likely play a role as well.
The effects of cannabis are similarly poorly understood to the origins of autism.
Although doctors caution against it, cannabis use has not been linked to miscarriages in humans (though animal studies have suggested an increased risk) and evidence on the link between weed and low birth-weight is mixed.
Marijuana use during pregnancy has been linked, however, to up to 2.3 times greater risks of stillbirth.
The Ottawa Hospital study did not investigate how exactly marijuana use in pregnancy might lead to autism in a child, but scientists believe that the drug’s interaction with the so-called endocannabinoid system within the nervous system could play a role in the development of the behavioral condition.