A new study sheds light on how THC causes damage to cells in the growing brain.

Although it doesn’t compare to fetal alcohol syndrome, cannabis exposure in the womb can cause any number of problems.

Children whose mothers use marijuana during pregnancy have a higher risk of stunted growth and of developing ADHD, anxiety, and depression later in life.

According to the Substance Abuse and Mental Health Services Administration (SAMHSA), more than 10 percent of unborn children in the U.S. and Europe have been exposed to cannabis.

Cannabis is one of the oldest domestic crops known to man, having co-evolved with humans for millennia, and it’s probable that many ancient cultures used the drug. However, modern breeding and cultivation techniques have dramatically boosted the plant’s levels of the psychoactive chemical Tetrahydrocannabinol (THC). THC crosses the placenta very easily, so when a pregnant mother uses the drug, so does her child.

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The effects that THC can have on a developing fetus are highlighted in a research study published yesterday in The EMBO Journal. The study was conducted by a team from Sweden, Austria, Germany, Finland, the UK, and the U.S., and led by Professor Tibor Harkany at the Karolinka Institutet (KI) in Sweden.

Growing a baby from a single cell is an astonishingly complex task. As the fetal brain develops, each cell must grow, migrate to the correct place, form into the correct shape, and successfully make as many as 10,000 connections with other cells. To reach other cells, each nerve cell grows a long, thin stalk called an axon, the end of which fans out to form many links.

This process requires a carefully-timed, intricate cascade of chemical signals. And it turns out that endocannabinoid, a signaling chemical in the body that THC mimics, is one of these. When THC enters the body, it interferes with endocannabinoid’s actions, competing with it for binding sites on target cells and generally getting its way.

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To test THC’s effects on the developing brain, Harkany started by exposing pregnant mice to low doses of THC and then examining the brains of their pups.

“The way axons form, bundle, and grow towards their target is impaired,” he told Healthline. Taking a closer look, he found that the number of binding sites for endocannabinoid had increased, and that axons were more likely to clump together. “The growth cones—the motile end tips that guide directional growth—look…different,” Harkany said.

Mice aren’t a perfect model for humans, and we still don’t know how different their endocannabinoid system is from that of humans. To confirm his findings, Harkany would have to look at people.

Harkany gathered human fetuses that had been donated to science and tested them to see if they had been exposed to THC. The THC-exposed fetuses had lower body weights and smaller foot length. When he looked inside their brains, he found reduced levels of stathmin-2, a protein involved in learning and memory formation.

Although the brain differences caused by THC exposure are fairly subtle, Harkany warns that their minds’ inherent instability is what leaves children at greater risk for developing certain psychiatric conditions later in life.

“Abnormal [axon] organization, even if remaining latent for long periods, might be prone to ‘circuit failure’ if provoked,” he explained. “A ‘double hit’ scenario of failure, when a network advances into a runaway cascade upon a secondary insult, therefore might account for the increased incidence of schizophrenia, depression, and addiction in offspring prenatally exposed to cannabis.”

The take-home message, Harkany feels, is clear. “Cannabis should be avoided during pregnancy. And, if there is a medical indication for the mother then careful cost/benefit analysis should be conducted by medical professionals,” he said. “I appreciate the use of medical cannabis, but it should certainly be analyzed whether maternal benefits outweigh potential risk for the baby.”

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