Earlier this week, the American Academy of Pediatrics released a statement on the risks associated with smoking pot while pregnant or breastfeeding. Its authors warn that while “social media is used to tout the use of marijuana for severe nausea associated with pregnancy,” there is little evidence available as to the benefits of smoking marijuana while pregnant, and that it may place the “fetus at risk from these agents or drugs.”
While studies do show that marijuana passes through the placental barrier to the fetus, we don’t know much about what — if anything — happens after that, says Emily Oster, an economist and author of bestselling, highly praised Expecting Better: Why the Conventional Pregnancy Wisdom Is Wrong–and What You Really Need to Know. “We know that the chemical THC sticks around in breast milk, and crosses the placenta. Those things are true,” says Oster. “Does that mean it’s a problem? We don’t really know.”
Studies on cannabis use in pregnant or breastfeeding women are few and far between, and those that do exist are difficult to interpret, says Oster. Part of the difficulty lies in marijuana’s status as, until recently, an illegal drug — fewer women are willing to admit to using a stigmatized illegal substance, and those that do are likely to differ from those that don’t to the point that attributing those differences to pot use is grossly simplistic, says Oster. “You can compare women who smoke pot to those who don’t, and look at findings about their babies. There are a bunch of studies like that,” says Oster. “These studies are hard to interpret because the kind of people who admit to smoking marijuana are different than the kind of people who don’t — they tend to be younger, poorer, less likely to be married, but they’re also probably different in ways we don’t see.”
Even still, the evidence we do have suggests that there is no significant difference between the health outcomes of babies whose mothers smoked pot versus those whose mothers didn’t at the time of birth. “When researchers look at things about birth — so when women smoke pot during pregnancy, are their babies more likely to be premature, more likely to be small for gestational age, more likely to be low birthweight, they really don’t see any differences,” says Oster. There are some differences that seem to surface in these kids when they reach age 5 or 6, but again, to attribute them to their mothers’ pot use is, says Oster, “a reach.”
“When researchers do cognitive testing of kids [whose mothers smoked pot during pregnancy], there are studies that point to slightly worse performance on verbal tests or on [assessments] that look like hyperactive behavior, or ability to focus on a task,” says Oster. “These kinds of studies and outcomes are even more complicated to look at than something like low birth weight, because if there are differences across moms during their pregnancies, there are also going to be differences during their behavior during the kids’ childhood.” For instance, how do we know it’s pot during pregnancy that made a kid more hyperactive, and not the circumstances in which he was raised over the five years afterward?
The ideal, as with any experimental study, would be a controlled, randomized test, but randomly assigning marijuana to pregnant women is, obviously, unethical. The best we can hope for, says Oster, is more information. While the effects of mild or moderate alcohol consumption during pregnancy remain somewhat unclear, Oster says that the sheer breadth of information available has done a lot to improve our understanding of the risks. “There is much more evidence on alcohol and pregnancy than there is on pot — both more compelling evidence that having a lot of it is bad, and compelling evidence that having a little bit is not bad,” she says. “We just have a lot more data. Again, it’s of the same type, in the sense of not being randomized, and having a lot of the same biases, but there’s just way, way more of it.”
As legalization spreads, Oster expects more and better data to become available — and that a higher volume of data will help undercut those inherent biases between smoking and non-smoking groups. “Even though it will be true that there will still be differences [between groups], they will maybe be smaller than [now, when] one person is breaking the law and one person is not,” says Oster. Economists expect that pot use will increase among pregnant women in states that legalize marijuana, and that will allow for more accurate comparisons to be made: between pregnant women in states where cannabis is legal, and pregnant women in states where it’s not. “You’d basically look at whether babies’ birth weight goes down in states that legalize, relative to states that don’t, around the time of legalization,” says Oster.
Strategies like these will help us get closer to the truth, but Oster acknowledges that isolating the exact effects of marijuana will “always be hard.” As is so often the case in the social sciences, the best, most accurate answer we have is also an ambiguous one — when it comes to the effects of smoking pot while pregnant, Oster says, “We just don’t know.” That said, she believes it’s best, until we do have more information, to exercise caution. “My bottom line is I would not do it,” she tells me. “My guess is that the recommendations are over-cautious, but this is my bottom line.”