CHICAGO (CBS) — A new survey finds more women are using marijuana to treat morning sickness.

Marijuana is the most commonly used illicit drug during pregnancy, and its use is increasing, according to a letter published Tuesday in the Journal of American Medical Association (JAMA).

Marijuana use among U.S. pregnant women increased from 4.2 percent in 2009 to 7.1 percent last year.

Researchers say the findings are from self-reported surveys and likely underestimate use, which “leaves the scope of the problem unclear,” JAMA says.

“We investigated trends of prenatal marijuana use from 2009-2016 using data from a large California health care system with universal screening via self-report and urine toxicology.”

While the letter does not discuss any potential problems from pot use during pregnancy, the Centers for Disease Control and Prevention (CDC) advises pregnant women to avoid it.