The proportion of women there who used marijuana in some form while they were pregnant almost doubled between 2009 and 2016. "In aggregated 2002-2012 data, 14.6 percent of USA pregnant adolescents reported past-month use".

The new research from Kaiser Permanente suggests that women know that their doctors would not approve of their marijuana use: Less than half of the women that tested positive for marijuana copped to using it when asked in a survey.

The U.S. Centers for Disease Control and Prevention says the health effects of marijuana on a fetus could include low birth weight and developmental problems, according to CNN.

The study, published Tuesday in the Journal of the American Medical Association, observed a sample of pregnant women in California. From 2009 to 2016, marijuana use among moms-to-be jumped from 4.2% to 7.1%, the study said.

"Of concern" to the researchers was that fact that marijuana use was particularly common among younger pregnant women.

"Marijuana is [one of] the most commonly used illicit drugs during pregnancy, and its use is increasing", the study says.

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".

Medical use of marijuana is already legal in California, which was one of the first states to legalize marijuana for that use two decades ago.

The study also reported an increase in pot usage among older pregnant women ages 25 to 34.