A growing number of pregnant women are using marijuana, and the habit is expanding fastest among teens and young adults, a U.S. study suggests.

Among teen mothers under age 18, marijuana use during pregnancy surged from about 13 percent in 2009 to almost 22 percent in 2016, researchers found. Over that same period, the proportion of pregnant women aged 18 to 24 using marijuana rose from 10 percent to 19 percent.

Across all age groups, marijuana use during pregnancy increased from 4 percent at the start of the study to 7 percent by the end.

Nationally representative studies have found that the prevalence of marijuana use among U.S. adult pregnant women has increased over time; however, these studies have not included objective measures of biochemically verified marijuana use and likely underestimate the prevalence, said lead study author Kelly Young and senior author Dr. Nancy Goler, both of Kaiser Permanente Northern California in Oakland.