Study of California women finds 1 in 14 used pot during pregnancy


Prenatal exposure to pot has become increasingly common in California since the state legalized medical marijuana in 1996, according to a new study.

Pot use by women during the first two months of pregnancy increased by about 7.5% per year between 2009 and 2016, researchers reported this week in the Journal of the American Medical Assn. By the end of that period, about 1 in 14 women were using some form of marijuana after they had conceived.

Those numbers are based on medical records of nearly 280,000 pregnant women in northern California who were part of the Kaiser Permanente health system. The women were asked about their recent pot use during a prenatal exam about eight weeks into their pregnancies. Doctors also had patients take a cannabis toxicology test during their first trimester.


Overall, the prevalence of pot use rose from 4.2% in 2009 to 7.1% in 2016, Kaiser research scientist Kelly C. Young-Wolff and her colleagues found. That’s higher than figures reported previously — a study based on the National Survey on Drug Use and Health, for instance, found that 3.9% of pregnant women used marijuana in 2014.

But surveys tend to underestimate drug use. Indeed, 55% of the women in the Kaiser sample who used pot during pregnancy denied doing so on a questionnaire but came up positive on their toxicology test.

The biggest jump in prenatal pot use was seen among younger women. In 2009, 12.5% of women 17 or younger and 9.8% of women 18 to 24 used marijuana during pregnancy. By 2016, those figures rose to 21.8% and 19%, respectively.

But it wasn’t just women in their teens and early 20s — pot use increased significantly for all age groups during the seven years of the study, the researchers found. For women 25 to 34, it rose from 3.4% in 2009 to 5.1% in 2016. For women 35 and older, it rose from 2.1% to 3.3% during the same period.

It’s not clear that all of these prenatal pot users knew they were pregnant when they used marijuana, the study authors wrote. The toxicology test typically picks up cannabis use within the previous 30 days, and if the drug was particularly potent, it could stay in a woman’s system for a longer period of time.

Even if women didn’t intend to use pot while pregnant, their babies could still be at risk. Marijuana acts on the endocannabinoid system, which begins forming about 16 days after conception and plays an important role in brain development. Some health experts are concerned that prenatal pot exposure could interrupt this process.


A preliminary study from the Netherlands found that children exposed to marijuana in utero were more likely to experience fetal growth restriction. This condition can make it difficult for newborns to breathe or maintain their body temperature, and in extreme cases can cause stillbirth.

Much more research is needed to understand marijuana’s effect on the developing brain, and the study authors said that was especially true considering that the sale of pot for recreational use will become legal in California on Jan. 1.

“Continued monitoring of trends, exposure timing, and offspring outcomes is important as marijuana potency rises in an increasingly permissive legal landscape,” they wrote.

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