Teens and young adults reported a particularly high use of marijuana compared to women over 24.
Marijuana use among pregnant mothers in California rose from 4.2 percent to 7.1 percent from 2009-2016. In 2016, almost a quarter of pregnant teenagers had used marijuana, as had about one in five women between 18 and 24.
While only about 30 percent of the pregnant women in the study reported that they used marijuana in a survey, about 55 percent tested positive for it in a drug screen, though self-reporting did increase in line with use each year. "California legalized medical marijuana use in 1996, so they have grown up with the idea of it not only not being illegal but being a medical therapy", Dr. Horsager-Boehrer told CNN.
The committee opinion stated that while most women believe that smoking marijuana is safe during pregnancy, it should be discouraged because scientists still don't know exactly what it does to developing fetuses.
The American Congress of Obstetricians and Gynecologists recommends that women who are pregnant, planning to become pregnant, or are breastfeeding should avoid the drug because the effects of marijuana on unborn and newborn children are not well known.
The study, published Tuesday in the "Journal of the American Medical Association", observed a sample of pregnant women in California.
More pregnant women smoking pot
Whatever the reasons, the findings could be bad news for babies, since "initial evidence suggests that prenatal marijuana may impair fetal growth and neurodevelopment", Young-Wolff's group said. Beyond birth weight, she said, "data are really limited by the number and the quality of existing studies".
Young and Goler did not condone the consumption of marijuana during pregnancy. However, self-reported data is notoriously unreliable. Marijuana use during pregnancy increased from 2.37 percent to 3.85 percent in 2014. But even so, more mothers are willing to run the risk, it seems.
Young-Wolff said marijuana usage increased the most in young females.
And women may even be using marijuana after they took the survey-on goal.
"We are simply beginning to expose what's underneath regarding understanding cannabis use in pregnancy", said Dr. Marcel Bonn-Miller, an analyst at the University of Pennsylvania Perelman School of Medicine in Philadelphia who wasn't associated with the investigation.