Heavy cannabis use during pregnancy linked to disruption in brain growth.
Researchers using brain imaging gain rare insight into how prenatal exposure to modern, high-THC cannabis affects brain development into adulthood.
McGill University researchers at the Douglas Research Centre have found evidence that heavy cannabis use during pregnancy can cause delays in brain development in the fetus that persist into adulthood.
Using advanced MRI techniques, the team tracked the effects of prenatal cannabis exposure in mice across key developmental stages.
While public health agencies caution against cannabis use during pregnancy, most supporting evidence from humans is observational. The findings add biological evidence showing how heavy use can disrupt brain growth from early development to adulthood.
Published in Molecular Psychiatry, a Nature Portfolio journal, the preclinical study also reflects the higher-potency cannabis available today, helping to fill a gap in understanding its potential risks.
“Since cannabis legalization is relatively recent, we don’t yet have long-term human data on newer THC products,” said senior author Mallar Chakravarty, Full Professor in the Department of Psychiatry and researcher at the Douglas. “Our findings offer an early glimpse of possible outcomes a decade or two down the line.”
Tracking brain development over time
The average THC potency in dried cannabis has risen from about three per cent in the 1980s to roughly 15 per cent in 2022, with some strains reaching 30 per cent, according to Health Canada.
To model heavy use, researchers simulated daily exposure equivalent to one or two joints containing more than 10 per cent THC during a stage comparable to the first trimester of human pregnancy.
They observed developmental changes across three life stages:
Late pregnancy: Embryos exposed to THC had smaller bodies and larger brain ventricles that signal abnormal brain development.
Early life: Newborns gained weight faster, but their brains developed more slowly, suggesting a mismatch or delay.
Adolescence to adulthood: Smaller brain volumes persisted, especially in females, who also showed more anxiety-like behaviours.
“The good news is that many of these developmental delays are subtle and could likely be offset with a supportive environment,” said Chakravarty.