Prenatal Marijuana Exposure Was Linked to Altered Brain Activity in Young Adults 18-22 Years Later | Research

The takeaway

Young adults whose mothers used marijuana during pregnancy showed different brain activation patterns during executive function tasks, with consistently increased activity in left posterior brain regions despite performing tasks normally.

Read this if you are pregnant or considering pregnancy and use cannabis, or if you are a healthcare provider counseling pregnant patients.

18-22 years after prenatal marijuana exposure, brain activation patterns were still different during executive function tasks.

What the researchers found

Thirty-one young adults (ages 18-22) from the Ottawa Prenatal Prospective Study, a long-running longitudinal study, underwent fMRI during four executive function tasks: visuospatial working memory, response inhibition, verbal working memory, and interference control.

Sixteen had been prenatally exposed to marijuana while 15 had not. Both groups performed the tasks equally well behaviorally, achieving similar accuracy and speed.

However, brain blood flow patterns were significantly different. The prenatally exposed group consistently showed increased activity in left posterior brain regions across all four tasks. This pattern suggests the brain may be compensating, recruiting additional neural resources to achieve the same level of task performance.

The consistency of the left posterior activation increase across four different types of executive function tasks is notable, suggesting a broad rather than task-specific alteration.

Why it matters

This is one of the longest follow-up studies of prenatal marijuana exposure, showing that brain differences are detectable 18-22 years after exposure. While the prenatally exposed individuals performed tasks normally, the altered brain activation suggests they may be working harder to achieve the same results, which could have implications for cognitive reserve and performance under stress.

The numbers in context

31 participants aged 18-22. 16 prenatally exposed, 15 controls. Equal task performance across all 4 executive function tasks. Consistently increased left posterior brain activation in exposed group across all tasks.

How the study worked

Longitudinal cohort study from the Ottawa Prenatal Prospective Study (OPPS). 31 young adults (16 prenatally exposed, 15 controls) underwent fMRI during four executive function tasks. Prenatal marijuana exposure was documented during pregnancy through the OPPS protocols. Brain activation was compared between groups while controlling for behavioral performance.

What this study cannot tell us

Small sample size (31 participants). The study cannot control for all postnatal environmental differences between groups. Cannabis-using mothers may have differed from non-using mothers in other ways that affected child development. The OPPS cohort used cannabis products from the 1990s, which were lower potency than current products.

How to read the evidence

Moderate evidence from a unique long-term longitudinal cohort with fMRI data, though the small sample size and potential confounders limit the strength of causal conclusions.

When this study was published

Published in 2016 using participants from the Ottawa Prenatal Prospective Study. Modern cannabis products are significantly more potent than those used during these pregnancies.

The bigger picture

As marijuana use during pregnancy increases (some women use it for morning sickness), understanding the long-term developmental consequences is critical. This study shows that even when cognitive performance appears normal in early adulthood, the brain may be working differently, using compensatory mechanisms to maintain function.

Questions still open

Would the compensatory brain activation become insufficient under higher cognitive load, revealing performance deficits? Does the altered brain pattern affect cognitive aging or resilience to neurological disease? Would higher-potency cannabis products produce more pronounced effects? Do these brain differences have real-world functional consequences?

Common questions

Does prenatal marijuana exposure affect the brain long-term?

This study found that young adults whose mothers used marijuana during pregnancy showed different brain activation patterns 18-22 years later. They performed cognitive tasks normally but their brains worked differently, suggesting compensation. The long-term implications of this altered brain activity are not yet known.

If performance is normal, does it matter?

It might. Increased brain activation to achieve normal performance could mean the brain has less reserve capacity. Under high cognitive demands, stress, or with aging, this could potentially lead to difficulties that would not emerge under standard testing conditions.

Read the original research

Prenatal marijuana exposure impacts executive functioning into young adulthood: An fMRI study.

Neurotoxicology and teratology, 58, 53-59

View on PubMed ↗Publisher’s paper ↗

Citation

Smith, Andra M; Mioduszewski, Ola; Hatchard, Taylor; Byron-Alhassan, Aziza; Fall, Carley; Fried, Peter A. (2016). Prenatal marijuana exposure impacts executive functioning into young adulthood: An fMRI study.. Neurotoxicology and teratology, 58, 53-59. https://doi.org/10.1016/j.ntt.2016.05.010