A recent federal survey finds almost 4 percent of pregnant women said they had used marijuana in the past month in 2014, up from 2.4 percent in 2002.

Pregnant women who use marijuana often assume the drug has no effect on developing infants, The New York Times reports. But preliminary research suggests the main psychoactive ingredient in marijuana, THC, can cross the placenta and reach the fetus, experts tell the newspaper. This has the potential to harm brain development, cognition and birth weight. THC can also appear in breast milk, the article notes.

“There is an increased perception of the safety of cannabis use, even in pregnancy, without data to say it’s actually safe,” said Dr. Torri Metz, an obstetrician at Denver Health Medical Center who specializes in high-risk pregnancies. She says 10 percent of her patients admit they recently used marijuana.