The number of women using cannabis in the year before they get pregnant and early in their pregnancies is increasing, and their frequency of use is also rising, according to new data from Kaiser Permanente.

The research, published July 19, 2019, in JAMA Network Open, examined self-reported cannabis use among 276,991 pregnant women (representing 367,403 pregnancies) in Northern California over 9 years and found that cannabis use has increased over time.

From 2009 to 2017, the adjusted prevalence of self-reported cannabis use in the year before pregnancy increased from 6.8% to 12.5%, and the adjusted prevalence of self-reported cannabis use during pregnancy increased from 1.9% to 3.4% (rates were adjusted for demographics). Annual rates of change in self-reported daily, weekly, and monthly-or-less cannabis use increased significantly, though daily use increased most rapidly.

Among women who self-reported cannabis use during the year before pregnancy, the proportion who were daily users increased from 17% to 25%, and weekly users increased from 20% to 22%, while monthly-or-less users decreased from 63% to 53% during the study period. Similarly, among women who self-reported cannabis use during pregnancy, the proportion who were daily users increased from 15% to 21%, and weekly users from 25% to 27%, while monthly users decreased from 60% to 52%.

"These findings should alert women's health clinicians to be aware of potential increases in daily and weekly cannabis use among their patients," said lead author Kelly Young-Wolff, PhD, MPH, a research scientist with the Kaiser Permanente Division of Research. "The actual numbers are likely higher, as women may be unwilling to disclose their substance use to a medical professional."

In addition, the prevalence of daily and weekly cannabis use may have risen even further in the past year and a half following legalization of cannabis for recreational use in California in 2018, Young-Wolff said.

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The data come from women's initial prenatal visits at Kaiser Permanente in Northern California, which usually take place at around 8 weeks gestation, and do not reflect continued use throughout pregnancy. Investigators were unable to differentiate whether self-reported cannabis use during pregnancy occurred before or after women were aware that they were pregnant.

While the current findings are based on women's self-reporting, the results are supported by the Kaiser Permanente research team's December 2017 JAMA Research Letter showing an increase in prenatal cannabis use via urine toxicology testing. In this newer study, the authors focus on trends in frequency of use in the year before and during pregnancy.

Some women may use cannabis during pregnancy to manage morning sickness, the authors noted. The authors' previous work published in JAMA Internal Medicine in 2018 found women with severe nausea and vomiting in pregnancy were nearly 4 times more likely to use cannabis during the first trimester of pregnancy.

Women may get the impression from cannabis product marketing and online media that cannabis use is safe during pregnancy, said Young-Wolff. However, there is substantial evidence that exposure to cannabis in pregnancy is associated with having a low-birthweight baby, and the American College of Obstetricians and Gynecologists recommends women who are pregnant or contemplating pregnancy discontinue cannabis use because of concerns about impaired neurodevelopment and exposure to the adverse effects of smoking.

"There is still much that is unknown on the topic, including what type of cannabis products pregnant women are using and whether the health consequences differ based on mode of cannabis administration and frequency of prenatal cannabis use," Young-Wolff noted.

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More research is needed to offer women better, specific advice, said study senior author Nancy Goler, MD, an obstetrician/gynecologist and associate executive director of The Permanente Medical Group.

"There is an urgent need to better understand the effects of prenatal cannabis exposure as cannabis becomes legalized in more states and more widely accepted and used," Dr. Goler said. "Until such time as we fully understand the specific health risks cannabis poses for pregnant women and their fetuses, we are recommending stopping all cannabis use prior to conceiving and certainly once a woman knows she is pregnant."

The study was supported by a grant from the National Institute on Drug Abuse.

Young-Wolff and Kaiser Permanente Division of Research colleague Lindsay Avalos, PhD, MPH, have received a new 5-year grant from NIDA to support further research on maternal cannabis use during pregnancy. They plan to study whether prenatal cannabis use is associated with increased risk of adverse maternal, fetal, and neonatal outcomes using data from urine toxicology testing, self-reported frequency of prenatal cannabis use, and mode of cannabis administration. They will also test whether legalization of cannabis for recreational use in 2018 and local regulatory practices (such as retailer bans) are associated with variation in prenatal cannabis use.

Additional authors were Constance Weisner, DrPH, MSW, Varada Sarovar, PhD, Lue-Yen Tucker, Mary Anne Armstrong, MA, and Stacey Alexeeff, PhD, of the Kaiser Permanente Northern California Division of Research; and Amy Conway, MPH, of the Kaiser Permanente Northern California Early Start Program.