SAN FRANCISCO—The number of California women who used marijuana while pregnant increased substantially between 2009 and 2016, a new study shows.

Although most of the nearly 280,000 pregnant Californians who participated in the Kaiser Permanent study published Tuesday in the Journal of the American Medical Association did not test positive for pot or admit to using it, a small and growing number of them did.

The study tested more than 30,000 women during each year of the eight-year study, using questionnaires and analyzing urine toxicology tests.

Read more: U.S. border agents won’t stop seizing pot even after California legalizes it

In 2009, the first year of the study, 1,547 pregnant women were found to have ingested marijuana, or slightly more than 4 per cent of the 33,692 studied that year.

Last year, the number had grown to 2,588, or slightly more than 7 per cent of the 38,919 tested.

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.

Little is known about the effect of marijuana use on pregnant women, fetuses, and children after birth, the study concedes. Other studies have shown that it’s correlated to lower birth weights, and guidelines from the American College of Obstetricians and Gynecology recommend that pregnant women discontinue their use of the drug.

“The reason that this becomes worrisome to me as an OB/GYN is that there are no studies that have ever reassured me that it would be safe to use during pregnancy,” said Nancy Goler, a co-author of the new study.

She said she is interested in what happens to the babies as they grow up.

“Do they have increased rates of addiction later in life?”

The participating women were all patients in Kaiser Permanente’s California health care system. The questionnaire and the tests were conducted when they were about around eight weeks pregnant.

Researchers found that younger women used marijuana more than older women while pregnant: 22 per cent of adolescents screened positive for marijuana use, as did 19 per cent of adults aged 18-24.

This tracked with a rise in marijuana use in the general population, said the study’s lead researcher, Kelly Young-Wolff.

“Nationally, studies have indicated that marijuana use is increasing over time, and that marijuana use is increasing over time among pregnant women, but the prevalence of marijuana use during pregnancy is higher in this study,” Young-Wolff said.

The study is also notable because it includes the results of a toxicology test—an objective, universal standard.

“That’s probably the biggest difference between our study and other studies,” Young-Wolff said. She said that previous studies generally relied mostly on self-reported data.

It’s unclear why more women are choosing to use marijuana during pregnancy, a question this study didn’t seek to address.

Loading... Loading... Loading... Loading... Loading... Loading...

“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.

One reason the numbers could be so high is that marijuana is more readily available in California.

“Our state may be different because it was the first to legalize medical marijuana in 1996,” said Young-Wolff, who is preparing to study prenatal marijuana after it becomes legal for recreational use in California on Jan. 1.