Experts say anyone considering having a baby should err on the side of caution and abstain from pot—but a new study shows that not enough women are getting the message.

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Marijuana use has become more prevalent—and increasingly legal—across the country. But doctors are concerned about one group in particular that is getting high more often: women who are pregnant or could become pregnant.

In annual surveys conducted on roughly 15,000 women a year, the percentage of pregnant respondents who said they’d used marijuana in the past month rose from 2.4% in 2002 to 3.9% in 2014, according to a new Columbia University study—a relative increase of 62%. Prevalence was highest in the 18-to-25 age group, in which 7.5% of pregnant women reported past-month use in 2014.

That’s concerning, say the study authors, because prenatal exposure to marijuana has been associated with developmental and health problems in children—including low birth weight, anemia, and impaired executive functions like impulse control, visual memory, and attention.

“The current state of science, even though it’s in the beginning stages, does suggest that there are real risks associated with marijuana use during pregnancy,” says first author Quiana Brown, PhD, a postdoctoral research fellow at Columbia’s Mailman School of Public Health.

“As that research is developing, we want pregnant women to be safe,” Brown told Health. “You don’t want to take a risk of your child having a neurodevelopmental deficit.”

The new study also notes that, in general, more women of reproductive age are using pot today than in the past. For non-pregnant respondents ages 18 to 44, the percentage of past-month marijuana use rose from 6.3% to 9.3%. That’s a 47% relative increase from 2002 to 2014.

In 2015, the American College of Obstetricians and Gynecologists recommended that pregnant women, or those considering pregnancy, be screened for and discouraged from using marijuana. The College also discourages marijuana use while breastfeeding, due to insufficient evidence about potential effects on infants.

But not enough women are getting the message, says Brown. “This study calls into question the current prevention efforts,” she told Health. “Marijuana use is increasing in all women of reproductive age—and particularly in pregnant women—and that goes against current recommendations and it’s a major public health concern.”

These rate increases are still lower than those found in the overall population. Between 2001 and 2013, marijuana use more than doubled among adults in the United States, the study reports. Medical marijuana is now legal in 29 states and Washington, DC, and several states have legalized the drug for recreational purposes, as well.

In a press release, co-author and professor of epidemiology Deborah Hasin, PhD, said that while the prevalence of pregnant marijuana users isn’t high, the increase over time suggests that further monitoring and more research is needed.

Researchers from the National Institute on Drug Abuse weighed in with an accompanying editorial, noting that marijuana does have some research-backed medical benefits—including an ability to relieve pain and nausea in certain circumstances.

“However, some sources on the internet are touting marijuana as a solution for the nausea that commonly accompanies pregnancy, including the severe condition hyperemesis gravidarum,” they wrote. Some evidence also suggests that women are using pot for morning sickness during their first trimester, when the fetus is at greatest risk for potentially harmful effects.

The editorial authors say that more studies are needed to clarify the true effects of prenatal marijuana exposure, and to separate out these effects from those of alcohol, tobacco, other drugs, poor nutrition, or other factors that may be more common in women who use marijuana during pregnancy.

But for now, they concluded, doctors in a position to recommend medical marijuana should be mindful of the possible risks, and err on the side of caution. “Pregnant women and those considering becoming pregnant should be advised to avoid using marijuana or other cannabinoids either recreationally or to treat their nausea,” they wrote.

Brown agrees, and says there’s a message here for everyone. “Physicians need to be aware of this, and public health professionals need to be aware of this from a preventative, public-health standpoint,” she says. “And from an individual health standpoint, women who are pregnant or planning to become pregnant should be aware of these trends—and the potential harms that can result.”

And what about people who aren’t planning on having kids anytime soon? The general public shouldn't ignore the risks associated with marijuana, either, she adds. “Research shows that marijuana use is associated with adverse consequences in some users; however, the public increasingly sees marijuana use as safe,” she says. “This is a skewed view that is contrary to scientific evidence—and one that may be particularly harmful for pregnant women and women planning to become pregnant.”