Marijuana is enjoying a popularity boon at the moment. In 2010, it was the most commonly used illegal drug, and considering that more states are considering medical marijuana laws, and both Washington and Colorado just made legal its recreational use, those figures are bound to rise even more. But with more women — and men, too — using marijuana than before, the chances of using it during pregnancy increase, too. According to a new study, marijuana use during pregnancy could have dire consequences for the growing fetus’ brain.

While marijuana use has been shown effective in treating some conditions, and hasn’t really been proven harmful to adults, studies have shown that they can drastically alter the brains of adolescents and children. One study, for example, suggested that adolescents who used marijuana were more likely to develop psychiatric disorders like schizophrenia, or experience some other kind of cognitive deficit. If that’s how it affected children who’s brains are already mostly developed — but still developing — how could it affect fetuses? After all, their brains are still so early in the developmental stage that many neurons have yet to make essential connections.

To find out, researchers from the Karolinska Institutet in Sweden tested the effects of marijuana’s active ingredient, delta-9-tetrahydrocannabinol (THC), in the growing brains of both mouse and human fetuses. They injected THC into pregnant mice, grew mouse brain cells while exposing them to THC, and looked at the brains of human fetuses that were aborted by their mothers, who used marijuana while pregnant, LiveScience reported.

The tests showed that THC affected the development of the cerebral cortex in both mice and humans. Specifically, THC exposure led to lower levels of a protein called Superior Cervical Ganglion 10 (SCG10). The protein helps with the brain’s neuronal connectivity, as it binds neuronal axons. Essentially, THC exposure led to less-connected or disconnected neurons — a harmful effect for the baby, considering that the cerebral cortex is critical to so many functions, including memory, attention, and language.

For the baby, these faulty connections could have long-lasting results, such as neuropsychiatric diseases, though they might not appear right away, the researchers said. “Even if the THC only would cause small changes, its effect may well be sufficient to sensitize the brain to later stressors or diseases to provoke neuropsychiatric illnesses in those affected in the future,” Professor Tibor Harkany, the study’s leader from the Karolinska Institutet and the Medical University in Vienna, Austria, said in a statement. “This concerns also the medical use of cannabis, which should be avoided during pregnancy.”

It would make sense for any woman who is pregnant to abstain from marijuana. Just like alcohol and cigarettes, it too is a drug that affects the body’s function. And although Harkany’s study sheds light on one harm marijuana use poses to fetuses, it’s not the first to show harmful effects. A study from December last year also found that marijuana use during pregnancy could triple the risk of stillbirths, even when the mother didn’t use any other substance. Harkany also said the study focused on the effects of marijuana only during the period of fetal growth when neuronal connections are made, adding that marijuana is more likely to affect the likeliness of getting pregnant than fetal development, if used before pregnancy.

Source: Tortoriello G, Harkany T, Morris C, et al. Miswiring the brain: Δ9‐tetrahydrocannabinol disrupts cortical development by inducing an SCG10/stathmin‐2 degradation pathway. The EMBO Journal. 2014.