Marijuana and Pregnancy: A Second Look
A new study examining the effects of cannabis use on fetuses in utero has been published in the Journal of the American Medical Association. This research is notable for just how thorough it was. It may constitute the most comprehensive study on marijuana use during pregnancy to date.
Writing in The Atlantic, Emily Oster explains what makes this particular study so valuable.
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.
Second, the authors did about as well as seems possible in handling the “women who use marijuana are different” problem. The women who reported cannabis use were much more likely to be teenagers, were poorer, were more likely to be underweight, and were much, much more likely to smoke cigarettes. Fifty-eight percent of the marijuana users smoked cigarettes, versus only 8 percent of the nonusers. Tobacco use is well known to increase the risk of prematurity and various birth complications.
The authors dealt with the issue by using a matching technique: They matched users to nonusers with all of the same characteristics. Faced with a 16-year-old underweight tobacco smoker in the user category with no previous pregnancies, the researchers looked in their (much larger) sample of nonusers for another 16-year-old underweight tobacco smoker with no previous pregnancies. They (or rather, their computers) did a version of this for all the marijuana users. Individuals without a match were left out. Individuals with many matches were compared with the average of their matches.
The goal was to have the two groups look as similar as possible on all variables other than use of marijuana. Then they could go ahead and compare the two groups with some confidence that the effects they saw were due to the differences in marijuana use.
The findings are troubling for the growing number of women using cannabis during pregnancy. The researchers found that babies whose mothers used marijuana in pregnancy were significantly more likely to be born prematurely or underweight and require transfer to the neonatal intensive care unit (NICU).
The study is not perfect, says Oster. More research is warranted, particularly to determine if there are any adverse effects on child development later in life. But for now, the best course of action for pregnant women is to avoid cannabis — at least until more is known.