Researchers at the Ottawa Hospital Research Institute found that cannabis use during pregnancy may increase the risk of delivering the child early. The study, which was published Tuesday in the Journal of the American Medical Association, evaluated 661,617 pregnancies, among them 9,427 women who said they were cannabis users. Twelve percent of the pregnant cannabis users experienced preterm births, compared with only six percent of non-cannabis users.
Still, some caveats are in order. The researchers said that among those who used cannabis during pregnancy, there were plenty of additional risk factors: 59 percent used tobacco, while nearly 20 percent used alcohol. Another 11 percent said they also used opioids. Among the women who said they only used cannabis and no other substances, the rate of preterm birth stood at 9.1%.
“It may be that cannabis exposure is associated more strongly with early and moderate preterm births as opposed to very preterm births, which may have different risk factors including infection, pregnancy-induced hypertension, or incompetent cervix,” the authors wrote. “The risk of preterm birth associated with cannabis exposure was statistically significant in subgroups of women who only used cannabis and no other substances, and among women using tobacco. There was evidence to suggest that the association between reported cannabis use and preterm birth may be stronger within the subgroup of tobacco users, which is a known risk factor for preterm birth.”
The authors also noted “several” limitations of the study, saying that the data was derived from “from self-reports, routine care records, and physician disclosure,” and that such sources “may be influenced by social stigma, desirability bias, and fear of intervention by child protection or social services.” Still, those limitations notwithstanding, they said that “self-report does seem to be a reliable method for determining cannabis use during pregnancy in epidemiological studies.”
The study can be seen as part of a flowering of research into cannabis use that has dovetailed with the wave of legalization worldwide—including in Canada, where recreational pot use became legal last year.
“With recent legalization in Canada and the United States, coupled with evidence of the potential medical benefits of the cannabinoids cannabidiol and tetrahydrocannabinol (THC),” the authors of the study wrote, “it is anticipated that cannabis use may further increase including among pregnant women.”
But just as the authors acknowledge that more research is needed, so too have others called for more studies into the health effects of cannabis use. Last month, the Canadian Institutes of Health Research committed $24.5 million in support of cannabis research. And in April, Charles R. Broderick, an early investor in Canada’s cannabis industry, donated $9 million to Harvard and MIT to “fill the research void that currently exists in the science of cannabis.”