New research finds that people who use cannabis early in pregnancy are at increased risk of poor pregnancy outcomes

As more states legalize marijuana for recreational use, researchers are looking into the potential health effects of cannabis use during pregnancy.

In a new study to be presented today at the Society for Maternal-Fetal Medicine’s (SMFM) annual meeting, The Pregnancy Meeting—and published in the American Journal of Obstetrics & Gynecology—researchers will unveil findings that suggest that using cannabis in the first six to 14 weeks of pregnancy is associated with poor pregnancy outcomes that are closely related to the function of the placenta.

The placenta plays an important role during pregnancy, including providing an infant with oxygen and nutrients, removing harmful waste and carbon dioxide, and producing hormones that help a baby grow. Medicine, drugs, alcohol and nicotine can transfer from a pregnant person’s bloodstream to their baby through the placenta.

Researchers analyzed urine samples collected during the Nulliparous Mothers-to-Be (nuMoM2b) study, a large, diverse multicenter cohort of pregnant people. Of the 10,038 nuMoM2b participants, researchers included data from 9,257 pregnant people in their first trimester of pregnancy.

After doing a drug screen for marijuana, researchers discovered that 540 (5.8 percent) tested positive for marijuana, meaning those individuals most likely used cannabis in the first six to 14 weeks of their pregnancy. The study found that using cannabis early in pregnancy is associated with adverse pregnancy outcomes related to the placenta, such as poor fetal growth, stillbirth, and an increased risk of developing high blood pressure during pregnancy.

“We wanted to look specifically at cannabis use early in pregnancy because that’s when the placenta is forming, and a lot of information we currently have indicates that cannabis use does affect the placenta,” says the study’s lead author Torri D. Metz, MD, MS, a maternal-fetal medicine subspecialist and associate professor of obstetrics and gynecology at the University of Utah Health in Salt Lake City.

“With recreational marijuana use becoming legal in more states, we need better data because patients are interested in understanding the risk of cannabis use in pregnancy so they can make an informed decision.”

The next step, say researchers, is to look at ongoing cannabis use during pregnancy and whether “the window of exposure” matters.