(CNN) - More pregnant women seem to be using pot, sometimes to ease the nausea of morning sickness or heightened anxiety, and a new study suggests that this slight rise in marijuana use is most pronounced among those younger in age.
The prevalence of marijuana use among a sample of moms-to-be in California climbed from 4.2% to 7.1% from 2009 through 2016, according to a research letter published in the journal JAMA on Tuesday.
Among pregnant teens younger than 18, marijuana use climbed from 12.5% to 21.8%, and among women 18 to 24, marijuana use climbed from 9.8% to 19%, the researchers found.
That research involved only certain women in California, but a separate study of pregnant women across the United States, published in JAMA in January, found that those who reported using marijuana in the previous month grew from 2.37% in 2002 to 3.85% in 2014. The women were 18 to 44.
Doctors caution that the health effects of marijuana on a fetus remain unclear but could include low birth weight and developmental problems, according to the US Centers for Disease Control and Prevention. Many of the chemicals in marijuana, like tetrahydrocannabinol, known as THC, could pass through a mother's system to her baby.
The American Congress of Obstetricians and Gynecologists recommends that "women who are pregnant or contemplating pregnancy should be encouraged to discontinue marijuana use" and "to discontinue use of marijuana for medicinal purposes in favor of an alternative therapy."
Additionally, "there are insufficient data to evaluate the effects of marijuana use on infants during lactation and breastfeeding, and in the absence of such data, marijuana use is discouraged," according to the recommendations.