When reviewing the half million figure dataset, the study’s authors found 1.4% of children were born with autism spectral disorder. Children whose mothers used cannabis during pregnancy have an increased risk of developing autism, a new Canadian study found. The study’s conclusions do not signal a cause-and-effect relationship between cannabis use while pregnant and children developing autism. But that number rose to 2.2% among children whose mothers used marijuana while pregnant. Mark Walker. “This is concerning, because we know so little about how cannabis affects pregnant women and their babies.