Women in the United States are increasingly using marijuana during pregnancy, a recent study finds. The numbers are still small, but the National Institute on Drug Abuse (NIDA) says this trend raises concerns because of evidence linking marijuana use in pregnant or breastfeeding women with low birth weights and interrupted brain development in children, among other problems.

A study found almost 10 percent of marijuana users in the U.S. have used it for medical reasons, and 20 percent of those medical marijuana users live in states where medical marijuana is not legal. According to an annual drug survey, 3.85 percent of pregnant women reported recently using marijuana, compared with 2.37 percent in 2002. Marijuana usage among non-pregnant women also increased between those years, from about 6 percent to 9 percent, according to a surveys at Columbia University Medical Center.

Another drug survey from 2013-14 found about 30 million adults in the U.S. used marijuana, and only 6 percent said they only used it for medical reasons. Some women are choosing marijuana as a solution for morning sickness during pregnancy.

Compared with alcohol, marijuana does not pose the highest risk for birth defects among fetuses. Marijuana is still less accessible or socially accepted than alcohol, and is thus used less frequently on average. However, the damage done to a child by taking the drug while pregnant could be severe.

According to a Netherlands study, marijuana usage among pregnant women is strongly associated with cortical thickness in their children, suggesting detrimental changes in their brains as a result of the inhaled drug.

One reason for this might be not the drug itself, but the way it is consumed through smoking, the study points out.

“Compared with nonexposed children, we observed smaller global brain volumes in tobacco-exposed children, whereas this was not found in cannabis-exposed children,” the study says. “Additionally, compared with non-exposed control subjects, we observed thinner cortices throughout the brain in tobacco-exposed children, whereas in cannabis-exposed children, we observed thicker cortices but only in the frontal regions in both hemispheres.”

According to the survey data involved in the study, 74.1 percent of marijuana-exposed pregnant women also reported using tobacco during their pregnancies, suggesting the problem as one having to do primarily with drug inhalation.

“This high degree of co-occurrence of cannabis and tobacco exposure makes it very difficult to conclude that the observed effects in the cannabis-exposed children are driven by cannabis exposure only,” the study adds.

In a journal submission titled, The Risks of Marijuana Use During Pregnancy, NIDA director Dr. Nora Volkow urges physicians to use options other than marijuana to treat women with prenatal nausea.

“Although the evidence for the effects of marijuana on human prenatal development is limited at this point, research does suggest that there is cause for concern,” Volkow warns. “A recent review and meta-analysis found that infants of women who used marijuana during pregnancy were more likely to be anemic, have lower birth weight, and require placement in neonatal intensive care than infants of mothers who did not use marijuana. Studies have also shown links between prenatal marijuana exposure and impaired higher-order executive functions such as impulse control, visual memory, and attention during the school years.”

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Top image (Getty Images): People explore various cannabis paraphernalia during the Denver 420 Rally, the world's largest celebration of both the legalization of cannabis and cannabis culture, May 21, 2016 in Denver Coloroado.