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 four percent at the start of the study to seven percent by the end. Marijuana is the most commonly used illegal drug during pregnancy, researchers note in JAMA. Some previous research suggests that prenatal marijuana exposure may impair fetal growth and neurodevelopment. For the study, researchers examined data on more than 279,000 pregnant women who were treated at Kaiser Permanente facilities that did universal screening for marijuana use as part of standard prenatal care. Women typically completed questionnaires asking about drug use when they were around eight weeks pregnant and then got lab tests during the same checkup or within the next two to four weeks. Drug tests sometimes revealed marijuana use that patients didn’t disclose on the questionnaires, and some women who reported drug use passed the screening tests, the study found. Among women who screened positive for marijuana use, 55 percent failed the lab tests but denied drug use on the questionnaires. Another 16 percent of women passed the drug tests but disclosed marijuana use on the questionnaires.