Its been known for years now that pregnant women smoking and imbibing other harmful substances can put their fetus risk. A new study today shows just how serious that risk can be.

The study, published in the journal Obstetrics & Gynecology, shows that smoking and illegal drug use more than doubles the risk of a stillbirth. This applies not just to tobacco smoke, but to marijuana as well.

“Smoking is a known risk factor for stillbirth, but this analysis gives us a much clearer picture of the risks than before,” said Dr. Uma Reddy, senior author of the study and a researcher at the National Institute of Child Health and Human Development (NICHD). “Additionally, results from the latest findings also showed that likely exposure to secondhand smoke can elevate the risk of stillbirth.

“With the legalization of marijuana in some states, it is especially important for pregnant women and health care providers to be aware that cannabis use can increase stillbirth risk.”

The study found that tobacco use by pregnant women increases the risk of stillbirth anywhere from 1.8 to 2.8 times, depending on how heavy the smoking is. Twenty percent of the women in the study tested positive for nicotine late in their pregnancies – double the ten percent of women who did not experience stillbirth and tested positive for nicotine.

Second hand tobacco smoke exposure meant a 2.1 ties greater risk of stillbirth, and Marijuana use correlated to 2.3 times greater risk of stillbirth. Women who had used marijuana, stimulants, or prescription painkillers were found to have a 2.1 times greater risk of stillbirth.

The study looked at women who had experienced stillbirth at hospitals across the U.S. Stillbirth in the study was defined as when a a fetus dies at or after 20 weeks of pregnancy. Researchers tested blood samples and the umbilical cords of these women, looking for the chemical byproducts of nicotine and other drugs.