As more states legalize marijuana, usage rates are going up, and so are questions about the plant’s health benefits or risks, including during pregnancy. Many women of childbearing age use marijuana recreationally, and wonder about continuing occasional use when they become pregnant.
When people ask me whether marijuana poses a risk to the fetus—which they do, frequently—my stock answer has been, “Honestly, we do not really know.” The characteristics of women who use marijuana differ from those who do not in ways that relate strongly to infant outcomes; this is especially true when marijuana is illegal, but it remains true after legalization. And collecting data on illegal (and stigmatized) activities is challenging. Most existing studies are small, poorly controlled, and have shown mixed results.
Which is why I was happy to see a new study published in the Journal of the American Medical Association last month on the specific question of how marijuana use in pregnancy may affect birth outcomes, including prematurity. This study, which uses data from Ontario, differs from existing work in a couple of ways.
First, it is enormous and comprehensive. The study includes more than 600,000 women—effectively all the women who gave birth in Ontario from 2012 through 2017—and the data come from administrative records. Infant outcomes were measured objectively, and information on marijuana use was collected in the same way for everyone in the sample: Women were asked about it at an early prenatal visit, and their answers were included in their official records. Use was self-reported, but, again, at least all the women were asked the same question.