The results of what Ottawa researchers suggest is the largest study of its kind have found that children of mothers who reported using cannabis during pregnancy had a 50% increased risk of developing autism spectrum disorder (ASD), compared with children who weren’t exposed to cannabis in utero, even after controlling for confounding factors.
The new results highlight that women who are thinking of using cannabis during pregnancy should be aware of the potential risks of the drug, and talk to a healthcare provider. “In the past, we haven’t had good data on the effect of cannabis on pregnancies,” said Daniel Corsi, PhD, epidemiologist at the Ottawa Hospital and BORN Ontario, which is affiliated with the CHEO Research Institute. “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.”
Corsi is first author of the team’s paper, which is published in Nature Medicine, and titled, “Maternal cannabis use in pregnancy and child neurodevelopmental outcomes.”
Recreational use of cannabis is now legalized in Canada, and expectant parents may think that cannabis can be used to treat morning sickness. However, legalization of cannabis doesn’t mean it’s safe for people who are pregnant or breastfeeding. Health Canada and the Society of Obstetricians and Gynaecologists of Canada recommend against these populations using cannabis, and health warnings to this effect appear on cannabis packaging.
Cannabinoids, including tetrahydrocannabinol in cannabis, readily cross the placenta and can enter the fetal bloodstream, the authors commented. Human and animal studies suggest that disruption of endocannabinoid signaling may interfere with normal neuronal wiring, and this could have implications for fetal neurodevelopment. “Exposure to cannabinoids while in utero can disrupt the fetal endogenous cannabinoid signaling system, which has several roles in embryo development.”
While previous studies have indicated that maternal cannabis use during pregnancy is linked with decreased concentration and attention in their offspring, “… data on long-term follow-up of children with exposure to cannabis in utero are currently limited,” the team continued. “There is a need for larger studies that can adequately control for confounding in cannabis-outcome associations.”
“Despite these warnings, there is evidence that more people are using cannabis during pregnancy,” said Mark Walker, MD, 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.”
To look for any association between cannabis exposure in pregnancy and neurodevelopmental outcomes in childhood, the research team turned to the BORN birth registry, and reviewed data from every birth in Ontario between 2007 and 2012, before recreational cannabis was legalized. Of the half a million women in the study, about 3,000 (0.6%) reported using cannabis during pregnancy.
The researchers had previously found that cannabis use in pregnancy was linked with an increased risk of preterm birth. In that study, they saw that women who used cannabis during pregnancy often used other substances including tobacco, alcohol, and opioids. So, for the study reported now in Nature Medicine, the researchers specifically looked at 2,200 women who reported using only cannabis, and no other substances, during pregnancy.
Their results showed that babies born to this group still had an increased risk of autism compared with those who did not use cannabis. The incidence of ASD was 4 per 1000 person-years among children exposed to cannabis in pregnancy, compared to 2.42 among unexposed children. “… the primary association between maternal cannabis use and ASD persisted in sensitivity analyses by other substance use, income, and preterm birth,” the team reported.
Interestingly, there was also a tentative link between prenatal cannabis exposure and an increased risk for children developing intellectual disabilities, learning disorders, and ADHD. However, the associations were smaller in magnitude—11–22%—than those between maternal cannabis use and offspring autism, and “did not attain statistical significance at conventional levels are matching and covariate adjustment,” the authors stated.
The researchers don’t know how much cannabis the women were using, how often, at what time during their pregnancy, or how it was consumed. They also noted that while they tried to control for other factors that could influence neurological development, their study showed an association, and could not demonstrate cause-and-effect. “Although findings of an increased risk for childhood neurodevelopmental disorders are of substantive interest, we emphasize a cautious interpretation given the likelihood of residual confounding,” they wrote.
As cannabis becomes more socially acceptable, healthcare researchers are aware that some parents-to-be might think it can be used to treat morning sickness. The Ottawa team suggests that women who are thinking about or currently are using cannabis during pregnancy should talk to their healthcare provider to help make an informed choice about what is best for them and their baby. And while the researchers acknowledged a number of limitations to their study, they nevertheless concluded, “In this large retrospective cohort, we found that children with mothers who reported cannabis use in pregnancy were at higher risk for ASD diagnosis … Further study is needed on the amount and timing of cannabis use in pregnancy and childhood health outcomes and following the legalization of cannabis in many jurisdictions.”