Researchers found that marijuana consumption among pregnant women in the state has risen from 2009 to 2016 from 4.2 percent to 7.1 percent.
The letter said, "use among females younger than 18 years to age 24 years of age increased the most, from 12.5 percent to 21.8 percent for those younger than 18 years, and from 9.8 percent to 19 percent for those aged 18 to 24".
Women who participated in the study were asked to self-report their marijuana use in a survey and undergo urine testing.
They also note that "research shows that using marijuana while pregnant can cause health problems in newborns-including low birth weight-".
"Marijuana use in pregnancy may be on the rise in part because the legalization of medical marijuana has made people think of the drug as less risky, even during pregnancy", Barbara Yankey, a public health researcher at Georgia State University, told NBC News. Numerous chemicals in marijuana, like tetrahydrocannabinol, known as THC, could pass through a mother's system to her baby. "So I think the idea that use is rising is just because of the greater legal exposure to marijuana that women have today versus 20 years ago".
California was the fist state in the USA to legalize medical marijuana in 1996.
Overall, there was an increase in cannabis use among study subjects, from four percent to seven percent, over the seven-year period.
California's initiative requires marijuana products to bear labels warning of potential health hazards, including that marijuana use while pregnant or breastfeeding could be harmful. However, self-reported data is notoriously unreliable. In aggregated 2002-2012 data, 14.6% of U.S. pregnant adolescents reported past-month use. But even so, more mothers are willing to run the risk, it seems. A total of 23 states have laws that allow the use of marijuana for medicinal purposes to some extent. Anecdotally, Young-Wolff said, some women may take marijuana while they're pregnant to deal with morning sickness.