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The number of moms-to-be who smoke pot while pregnant is higher than we thought and only growing, a study published today in the Journal of the American Medical Association suggests. While smoking weed might seem like a perfectly harmless way to relieve stress in most cases, there’s still a lot we don’t know about if and how cannabis could affect fetuses in the womb.

To find out how common marijuana use might be among pregnant women, the study authors used anonymous medical records from some 30,000 women living in the state of California who were enrolled in a insurance plan belonging to Kaiser Permanente, the country’s single-largest HMO. As part of a normal prenatal check-up at 8 weeks pregnant, the women were asked to fill out a standard medical questionnaire and to voluntarily take part in a drug screening. That allowed the researchers to not only see how often pregnant women admitted to using pot, but also how many actually tested positive for it during their pregnancy.

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Researchers found that from 2009 to 2016 the percentage of pregnant marijuana users (whether self-reported or tested positive) almost doubled, from 4.2 percent to 7.1 percent. The numbers were even higher with younger women: By 2016, 22 percent of teen moms tested positive for pot, as did 19 percent of women between the ages 18 to 24. Less than half of women who tested positive for pot also self-reported it.

The upwards trend does square with earlier research, but it’s higher than previous estimates that only relied on self-reports. While that reluctance to self-report seems obvious given marijuana is still illegal in most states, it more importantly means we might have seriously underestimated the scale of any potential health risks using pot during pregnancy could cause.

While marijuana has long been the subject of baseless fear-mongering, there are still legit concerns over how dangerous it could be for young people whose brains haven’t fully developed, let alone those still unborn.

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Animal studies, along with some but not all human studies, have suggested that cannabis use during pregnancy could be linked to low birth weight and other health problems that occur soon after birth. Last year, however, a review of 31 studies found there was no clear links. Any effects other studies had found, it suggested, might have been caused by things like smoking tobacco, which is also common among marijuana users.

Despite their findings, though, the review authors didn’t endorse using marijuana during pregnancy, pointing out they only looked at the short-term effects. And even in Colorado, the first state to legalize pot recreationally, health officials still adamantly warn against doing so. Elsewhere, there’s still suspicion that a mother’s long-term, heavy use could cause bigger developmental problems for her children down the line, especially since the drug has only become more potent over the years.

As the authors of this current study note, while scientists are unsure about the risks of pot during pregnancy, the public definitely isn’t. They cite recent surveys showing that around 80 percent of pregnant women believe it’s risk-free. Given that California will become the latest state to legalize pot for all in 2018, that’s a pretty worrying disconnect.

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“In California, medical marijuana was legalized in 1996, and prenatal use may further escalate in 2018 when recreational marijuana is available legally,” the authors wrote.

[JAMA]