Use of these substances continues during pregnancy, especially among younger teens, the study found. More than one-third of all pregnant teens ages 12 to 14 said they used one or more substances in the previous month, the Austin American-Statesman reports.

Pregnant teens were most likely to use alcohol (16 percent), followed by marijuana (14 percent) and other illicit drugs (5 percent). The findings are published in Addictive Behaviors. The researchers found pregnant teens were less likely to use drugs or alcohol once they moved into their second or third trimester.

The study included data from 97,850 teen girls between ages 12 and 17. A total of 810 reported they were pregnant.

“To our knowledge, this is the largest study to date on the relationship between substance use and teen pregnancy,” study author Christopher Salas-Wright said in a news release. “Mothers’ substance use during pregnancy can have important consequences for the health and development of newborn babies. Despite efforts to prevent substance use among pregnant teens, our findings suggest that we still have a lot of work to do.”

The study found the risk of substance use was about 50 percent lower among pregnant teens who said they had parental support and limit-setting, as well as among those who had positive feelings about going to school. “This suggests that it makes sense to engage both parents and teachers in efforts to address substance use among pregnant teens,” said study co-author Michael G. Vaughn.