Moreover, the same report suggests that the number of people smoking weed in California, pregnant or not, may have increased and that using cannabis has become more acceptable in the state.
"We know some women say they use it to help with their vomiting and nausea, but we also know it's just being used more", Goler said. The survey contained questions about their use of marijuana within the past month.
The study is also notable because it includes the results of a toxicology test-an objective, universal standard.
Nevertheless, there were also women who stated that they hadn't used marijuana in the last month, although they had a positive urine test.
Researchers believe factors like marijuana legalization and morning sickness have contributed to the trend.
Short-Term Exposure to Low Levels of Air Pollution Deadly for Seniors
Women, non-white individuals and the very elderly had higher mortality rate increases associated with PM2.5 than other groups. Poverty, unhealthy lifestyle, or poor access to healthcare may play a role in such disparities, the researchers speculated.
This tracked with a rise in marijuana use in the general population, said the study's lead researcher, Kelly Young-Wolff.
The rate of marijuana use among pregnant women age 25 and older was considerably lower, at less than 5 percent.
"California is a little different in that we were the first state to legalize medical marijuana in 1996", Young-Wolff told Newsweek.
The American Congress of Obstetricians and Gynecologists has responded to the survey, saying that "women who are pregnant or contemplating pregnancy should be encouraged to discontinue marijuana use" and "to discontinue use of marijuana for medicinal purposes in favor of an alternative therapy".
A growing number of pregnant women are using marijuana, and the habit is expanding fastest among teens and young adults, a USA study suggests.
The study's authors are comparing only the per cent of marijuana users in each year, rather than the numbers, so they can control for age, ethnicity and household income. "Definitely, more research is needed".
For a national committee of physicians, the trends revealed by the California study appeared worrisome. "The effects of marijuana use may be as serious as those of cigarette smoking and alcohol consumption".