Numerous chemicals in marijuana, like tetrahydrocannabinol, known as THC, could pass through a mother's system to her baby.
There was one important caveat to the survey: Urine samples were taken at eight weeks of gestation, so pot use might have occurred before numerous women realized they were pregnant.
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.
Expectant California mothers apparently need a new bumper sticker: Bong on board.
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. Marijuana use during pregnancy increased from 2.37 percent to 3.85 percent in 2014.
"Of concern" to the researchers was that fact that marijuana use was particularly common among younger pregnant women. "We recommend (pregnant women) stop immediately".
But the researchers believe that urine samples are likely to provide the most accurate picture of marijuana use in pregnancy, because people tend to hesitate about admitting such behaviors. The study's authors said toxicology results show higher levels of marijuana than women disclosed in the survey, indicating women were under-reporting their marijuana use.
Pot usage among the women in the under-18 age bracket went up from 9.8 percent to 19 percent over the study period.
"Think about marijuana use from their perspective, especially in Northern California".
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.
"With the proximity to OR and Washington, they also have experience with any use being legal", she said.
From 2009 to 2016, marijuana use among moms-to-be jumped from 4.2% to 7.1%, the study said.
But doctors in the USA warn that drinking any alcohol while pregnant could come with medical risks, such as the possibility of miscarriage, stillbirth, or physical and behavioral problems in the baby.
Though little clear evidence exists on the effects of marijuana on developing human fetuses, animal studies have revealed that it may disrupt brain development. These women start folic acid, lose weight and reduce medication exposure of any kind before becoming pregnant.
In spite of its wide use, the new research suggests that pregnant women are still aware of some stigma against them for partaking in a toke or an edible.
"Then there's the other group who take a more pragmatic view of pregnancy".
Many women may not understand the potential risk, however.