Scientists have shown in a new study that one-time exposure during early pregnancy to cannabinoids (CBs) like THC or CBD can trigger growth abnormalities in the developing embryo. This was the first time that such a connection has been highlighted in the mammalian womb.

Scott Parnell, an assistant professor of cell biology and physiology at the University of North Carolina, administered either cannabinoids or cannabinoids with alcohol in varying amounts to female mice in their eight-day of pregnancy, which corresponds to 3-4 weeks of pregnancy in humans. This early pregnancy period is the most vulnerable for a developing embryo and is especially dangerous due to the fact that many women are not aware they are pregnant during this stage.

The study went on to show that the one-time use of CBD and THC, which are the primary ingredients in marijuana, caused brain and facial development effects similar to those experienced during fetal alcohol syndrome. When alcohol was administered together with either CBD or THC, the birth defects doubled.

“The development of the embryo in this time period is very similar across all vertebrates,” said Parnell in a statement. “In this study we also test a synthetic cannabinoid in zebrafish that yielded similar growth deformations as the natural CBs. Having the same results across animal models reinforces our findings.”

Parnell says that CBD and THC may be causing defects as a result of interactions at the cellular level that disrupt signaling between molecules and cells that control growth and development.

The CBD concentration was equivalent to what is considered a therapeutic range for humans, while the THC concentration was similar to that reached by a person smoking cannabis.

“The interaction between alcohol and CBs we witnessed is very concerning,” said the study’s first author, Eric Fish, PhD, research associate in the UNC School of Medicine Bowles Center for Alcohol Studies. “Previous studies have shown that CBs and alcohol are frequently used together, and for pregnant women we’re learning that could be very dangerous to a developing child.”

In the future, the researchers would like to run more tests but for now, the findings are worrisome.

According to previous research, marijuana use has not been associated with birth defects, stillbirth or preterm birth. This, in itself, is good news for pregnant women who have decided to take CBD oil, but this is by no means an endorsement. The absence of evidence is not evidence of absence. For instance, because cannabinoid receptors are involved in brain development, some fear that CBD oil might trigger growth abnormalities in the developing brain — which is exactly what he new study showed. However, others believe the opposite effect could be true — that is promoting healthy fetal brain development — since CBD can promote neurogenesis.

“We know that there is no safe period to drink alcohol during a pregnancy, and I think this research shows the same is likely true of marijuana use,” Parnell said.

The study appeared in the journal Nature Research.