Brain Scans of Young Adults Exposed to Cannabis Before Birth Show Lasting Changes | Research

The takeaway

Young adults (18-22) whose mothers used marijuana during pregnancy showed increased brain activation during response inhibition tasks and made more errors, even after controlling for their own current drug use.

Read this if you are pregnant or planning to become pregnant and considering cannabis use.

Prenatal marijuana exposure effects visible in brain scans 18-22 years later

What the researchers found

Using fMRI, 31 young adults from the Ottawa Prenatal Prospective Study showed that greater prenatal marijuana exposure was associated with increased neural activity in bilateral prefrontal cortex and right premotor cortex during response inhibition tasks. Activity in the left cerebellum was attenuated. Prenatally exposed participants made significantly more commission errors (failures to inhibit responses), though all performed above 85% accuracy.

These findings persisted when controlling for current marijuana use and prenatal exposure to nicotine, alcohol, and caffeine, suggesting the effects were specifically attributable to prenatal cannabis exposure and lasted into young adulthood.

Why it matters

This was one of the first fMRI studies to demonstrate lasting neural effects of prenatal cannabis exposure into adulthood. The finding that prenatally exposed young adults needed more brain activation to perform impulse control tasks suggested persistent, subtle neurological changes from in-utero exposure.

The numbers in context

Thirty-one participants aged 18-22 from the OPPS. Increased activation in bilateral prefrontal cortex and right premotor cortex. Decreased activation in left cerebellum. More commission errors in exposed group, all above 85% accuracy.

How the study worked

This was a longitudinal study using fMRI within the Ottawa Prenatal Prospective Study (OPPS), which had followed participants from birth for over 20 years. Thirty-one participants aged 18-22 performed a Go/No-Go task during fMRI scanning. Analyses controlled for current drug use and prenatal exposure to other substances.

What this study cannot tell us

The OPPS cohort is relatively small and non-randomly selected. Self-reported prenatal drug use may be inaccurate. The cross-sectional fMRI assessment cannot prove the brain differences were caused by prenatal exposure rather than other factors. Controlling for current drug use does not eliminate all confounding.

How to read the evidence

This is a longitudinal fMRI study from a well-established cohort with 20+ years of data, providing moderate-level evidence.

When this study was published

Published in 2004. The OPPS cohort has continued to be studied, and prenatal cannabis exposure research has expanded.

The bigger picture

The OPPS is one of the longest-running studies of prenatal cannabis exposure. These findings contributed to public health recommendations against cannabis use during pregnancy and demonstrated that prenatal effects can persist into adulthood, long after the exposure occurred.

Questions still open

Do these neural differences translate to real-world functional impairments? Would the effects be different with modern, higher-potency cannabis? Could the increased brain activation represent successful compensation rather than impairment?

Common questions

Can using marijuana during pregnancy affect the baby's brain long-term?

This study found brain activation differences in young adults whose mothers used marijuana during pregnancy, persisting 18-22 years after exposure. The affected brain regions are involved in impulse control and decision-making.

Were the prenatally exposed young adults impaired?

They made more errors on impulse control tasks and their brains worked harder (more activation), but all performed above 85% accuracy. This suggests subtle changes in brain efficiency rather than obvious impairment.

Read the original research

Effects of prenatal marijuana on response inhibition: an fMRI study of young adults.

Neurotoxicology and teratology, 26(4), 533-42

View on PubMed ↗

Citation

Smith, Andra M; Fried, Peter A; Hogan, Matthew J; Cameron, Ian. (2004). Effects of prenatal marijuana on response inhibition: an fMRI study of young adults.. Neurotoxicology and teratology, 26(4), 533-42.