Teens and young adults reported a particularly high use of marijuana compared to women over 24. 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. Over that same period, the proportion of pregnant women aged 18 to 24 using marijuana rose from 10 percent to 19 percent.
"Marijuana is the most commonly used illicit drugs during pregnancy, and its use is increasing", the study said. According to one survey conducted between 2007 and 2012, 79 percent of women "reported perceiving little to no harm in prenatal [pot] use", the Kaiser team noted. "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".
Researchers relied on self-administered questionnaires completed by 279,457 women eight weeks into their pregnancy.
The proportion of women there who used marijuana in some form while they were pregnant almost doubled between 2009 and 2016.
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.
Medical marijuana was legalized in California in 1996, and doctors say some patients use marijuana to self-treat the nausea and vomiting associated with pregnancy or to stimulate appetite.
A committee opinion from the American College of Obstetricians and Gynecologists said: "In the absence of such data, marijuana use is discouraged". California will make recreational marijuana legal in 2018.
Barbara Yankey, a researcher at Georgia State University, told Reuters marijuana use may be on the rise because of the recent legalization of its recreational use "has made people think of the drug as less unsafe, even during pregnancy".
And women may even be using marijuana after they took the survey-on goal.