There is also a notable uptick from 12.5 percent to 21.8 percent in the use of marijuana among pregnant Californian teens younger than 18, the researchers found.
"Marijuana is [one of] the most commonly used illicit drugs during pregnancy, and its use is increasing", the study says.
Still, the American College of Obstetricians and Gynecologists recommends that obstetrician-gynecologists counsel women against using marijuana while trying to get pregnant, during pregnancy, and while they are breastfeeding.
In 2009, only about four percent of pregnant women were using marijuana.
For other age groups, the researchers found that marijuana use rose from 3.4% to 5.1% among women 25 to 34 and from 2.1% to 3.3% among women older than 34.
"Our study is important because it addressed key limitations of prior studies by investigating trends in prenatal marijuana use using data from a large California health care system with gold standard universal screening for prenatal marijuana use", Goler and Young told Reuters.
Prenatal exposure to marijuana has been associated with low birth weight, impaired brain development, and effects on behaviour.
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.
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.
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.