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.

Ultimately, the authors did find some increased risk of poor birth outcomes among the cannabis users: most notably an increased risk of premature birth, an increased risk of the infant being small for his or her gestational age, and an increased risk of transfer to the NICU. These risks were large if not staggering. For example, the risk of preterm birth was 10 percent in the cannabis user group versus 7 percent among nonusers. That’s a statistically significant difference.

This study could change the tenor of the discussion around marijuana and pregnancy. Already the website UpToDate, a resource for doctors to get up-to-the-minute advice about treating patients, suggests that medical professionals urge greater caution in light of these findings. That change seems appropriate. The JAMA study represents a sizable advance over what we knew before.

However, this new study is not perfect or comprehensive, and it shouldn’t be the end of the story. For one thing, despite the matching effort, it may be that the marijuana users in the sample were different from nonusers in ways that were unobservable to the researchers. Put differently, users are more likely to be underweight—could it be that they are less healthy in other respects, too? Since the researchers did not observe other metrics of health, they could not use their matching program to hold those constant. Nor did the authors offer answers to important questions such as: Does it matter when in pregnancy the marijuana use occurs? Does it matter if the user smokes marijuana or prefers edibles? And on and on. The authors, moreover, did not look into how marijuana use in pregnant women affects a child’s brain development later in life.