Marijuana use during pregnancy may be linked to increased risk of autism


In the wake of data showing that marijuana use among pregnant women in the U.S. more than doubled from 2002-2017, a new Nature study is suggesting that babies born to moms who use cannabis during pregnancy may be at higher risk of developing autism.

The study, carried out by the Ottawa Hospital Research Institute, looked at live birth data in Ontario, Canada from 2007-2012, studying a total of 508,025 births. Out of this group, nearly 2,200 women reported marijuana use only (i.e. no other substances) during pregnancy. The researchers found that among this group, the rate of autism was 4 per 1,000 person-years compared to 2.42 among those who were not exposed to cannabis in utero. In other words, babies born to moms who used marijuana were 1.5 times more likely to have autism than those who did not.

The study, as many have since pointed out, does not establish a causal relationship between marijuana and autism. Furthermore, the authors are quick to note that the data is limited — lacking information about which trimesters the cannabis use occurred, how frequently it was used and in what way it was consumed (i.e. edible, smoking or vaping). The small sample size, taken a year before Canada legalized marijuana, may also affect the results.

On top of that, the data lacks information on genetic components of the mothers or environmental risk factors, which researchers believe play a major role in autism.

Still, even with these limitations, the findings are bolstered by previous studies which have shown that marijuana has the potential to negatively impact a growing fetus. According to the Centers for Disease Control and Prevention, marijuana use during pregnancy has been linked with a variety of conditions including low birth weight, hyperactivity and inattention (experts have said more studies on all are needed).

Further research has shown that the active chemicals in cannabis can cross the placental wall and impact a fetus’ brain development. In 2016, the American Academy of Pediatrics (AAP) released a specific report on marijuana use during pregnancy detailing the available evidence and concluded that, given all of these concerns, it was unsafe.

Given this, Dr. Seth Ammerman, one of the AAP report’s co-authors, says that the findings from Ottawa aren’t necessarily surprising. “The bottom line is that there is only limited evidence, but the limited evidence is that marijuana use during pregnancy does seem to have the potential to cause problems with abnormal development — not only in infancy but potentially in childhood, adolescence and even young adulthood,” Ammerman, an adolescent and addiction medicine specialist and retired Stanford University professor, tells Yahoo Life. “The recommendations are to not use marijuana during pregnancy.”

Does marijuana use during pregnancy increase the risk of autism? Experts unpack what we know thus far. (Photo: Getty Images)
Does marijuana use during pregnancy increase the risk of autism? Experts unpack what we know thus far. (Photo: Getty Images)

While there is existing evidence showing cannabis may lead to developmental problems, Margaret Daniele Fallin, professor at Johns Hopkins University and director of JHU’s Wendy Klag Center for Autism & Developmental Disabilities, urges caution when drawing conclusions. “Marijuana use in pregnancy has not been extensively studied as a risk factor for autism,” Fallin tells Yahoo Life. “Prenatal exposure to substances is a plausible risk factor for neurodevelopmental problems, so it is important to continue asking this and related questions ... This study is a good early step. However, given it is not a randomized controlled trial (which would likely not be ethically reasonable), we need to build a body of research across multiple well-designed and implemented studies to make conclusions about marijuana during pregnancy and risk for autism in the baby.”

Ammerman agrees that more research is needed, but suggests that marijuana use during pregnancy could be viewed through the lens of another drug. “It’s probably similar to alcohol — but again very limited evidence — whereas the first trimester would be the most problematic, second trimester somewhat less so and the third trimester somewhat less than second or first, just because of fetal development and how that works,” Ammerman says. “But your first trimester when everything is really starting to develop, if you bring cannabis into the picture and the fetal brain is developing, you're bringing in all these external cannabinoids ... we really want to limit an external exposure to that developing brain.”

He notes, however, that there may be a variety of “confounding variables” that may not have to do with marijuana but may independently be making the situation worse. “Sometimes when we looked at the studies there might be other substance use as well, which could be problematic,” Ammerman says. “Or it could be prenatal care that's not regular, particularly for underserved populations.”

Dr. Darine El-Chaâr, a maternal-fetal medicine physician at the Ottawa Hospital and one of the study’s authors, says that many have been dismayed by the conclusion. “There’s been people who are pretty upset about the results ... and listen, it's still a topic that's touchy,” El-Chaâr tells Yahoo Life. “So what I say is, I’m not here to judge or put a stigma on anyone who uses cannabis. Women who use cannabis usually have a reason for it. And we try to work together to change that if possible.”

As a practicing physician, talking to women about cannabis use during pregnancy is something El-Chaâr does regularly. And although she sympathizes with those who use it, she says there are other, safer methods available. “What I tell my patients is that just because it's legal and a natural substance doesn't mean it's safe, right?” El-Chaâr says. “There are other substances in pregnancy that are illegal and from natural sources — like alcohol — that are not safe in pregnancy. So far the data indicates that there is placental transfer and affects the brain. So if you find it helps with morning sickness, we can find other medications that could help. But currently, we are recommending to change the treatment, because we don't have any better evidence that it is safe.”

Fallin adds that the study, while important at “provoking further research,” should not be used to condemn pregnant women. “I do worry that we as a society put a lot of pressure on women during pregnancy to ‘do this’ but ‘not do that,’” Fallin says. “It can be confusing and anxiety-provoking for women. A critical message is for women to take care of the mental and physical well being. Stay informed, try to minimize risk while also not becoming anxious about every activity and decision.”

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