The study, from the Journal of the American Medical Association (JAMA), notes that from 2009 to 2016, pot use among expectant mothers jumped from 4.2 percent to 7.1 percent.
There's been a rise in the number of pregnant women who are using marijuana to soothe anxiety and morning sickness.
California's initiative requires marijuana products to bear labels warning of potential health hazards, including that marijuana use while pregnant or breastfeeding could be harmful.
Some of the sharpest increases in marijuana usage were among pregnant women under the age of 18 and between the ages of 18 to 24 years old.
"Marijuana is the most commonly used illicit drug during pregnancy, and its use is increasing", the study said. A total of 3.3 percent of women older than 34 also reported to using the drug during their pregnancy in 2016, an increase from 2.1 percent in 2009.
"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.
However, the institute says more research must be done to determine which effects are related to marijuana exposure and which are related to environmental factors that could be associated with the mother's marijuana use, such as poverty or other drug use. "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", said Horsager-Boehrer, who was not involved in the study.
"Marijuana is detectable in urine approximately 30 days after last use and this varies with heaviness of use and marijuana potency", she said.
"I have found that women frequently fall into two groups during pregnancy".