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.
A rare look across the lifespan
The methods used provided a level of detail not often achieved in preclinical studies, the researchers explain.
"That's partly because this type of research is incredibly resource intensive," said first author Lani Cupo, who carried out the work over six years during her PhD at McGill. "We used live brain imaging to follow development across the lifespan, which isn't commonly done in mice."
Collaborators at the University of Victoria later used ultra-high-resolution microscopy to examine how brain cells changed after THC exposure.
Supporting informed choices
The researchers note that some people use cannabis before realizing they are pregnant, while others use it to manage nausea or to cope with anxiety and depression, conditions that can also affect pregnancy outcomes.
"There is no 'ideal' pregnancy," said Chakravarty. "This isn't about what is good or bad, it's about giving people the information they need to make informed decisions."
A follow-up study will explore whether other forms of cannabis, such as edibles, vaping and CBD products affect the brain differently.
About the study
"Impact of prenatal delta-9-tetrahydrocannabinol exposure on mouse brain development: a fetal-to-adulthood magnetic resonance imaging study" by Lani Cupo and Mallar Chakravarty et al., was published in Molecular Psychiatry. It was supported by the Canadian Institutes of Health Research.
