Researchers find biological evidence for the impact of prenatal cannabis exposure on mental health

Scientists are trying to understand how cannabis may affect the long-term neurological development of babies when exposed to it in the womb.

Previous research by WashU researchers Sarah Paul and David Baranger in the Behavioral Research and Imaging Neurogenetics (BRAIN) lab led by Ryan Bogdan found a link between cannabis exposure during pregnancy and possible mental health conditions in childhood and adolescence. However, it remains unclear what biological mechanisms may explain this association.

In research published in Nature Mental Health This month, Bogdan, Dean’s Distinguished Professor of Psychological & Brain Sciences at Washington University in St. Louis, and postdoctoral researcher Baranger outline some of those potential mechanisms, the intervening biological steps that may play a role in how prenatal cannabis exposure leads to behavioral problems later in life.

“We see evidence that cannabis exposure can impact the developing brain, which is consistent with the association with mental health,” Baranger said.

Trying to figure out the long-term effects of cannabis exposure during pregnancy is not an easy knot to untangle. There are many confounding factors that influence mental health and behavior.

For example, let’s say someone is exposed to cannabis in the womb and later develops attention deficit disorder as a teenager – how do you distinguish that as a heritable trait, or a trait that is influenced by environmental factors, versus a trait that was influenced by cannabis exposure early in development? Or all three processes might contribute to eventual psychopathology.

Another complication is the increasing prevalence of cannabis use, including among pregnant women. Between 2002 and 2017, cannabis use increased from 3% to 7%.

Researchers used statistical methods to filter out some of these confounding factors and to provide potential biological measures of the relationship between cannabis use during pregnancy and adolescent behavior.

There’s nothing that can prove 100 percent causation. “But we can look at the plausibility of the causal relationship and identify potential biological correlates that are associated with cannabis exposure and mental health outcomes, which suggests that it is plausible,” Bogdan said of the study results.

Researchers used data from the Adolescent Brain and Cognitive Development (ABCD) Study, an ongoing research project involving nearly 12,000 children in the United States.

As part of that study, researchers collected data on each mother’s substance use before birth and neuroimaging data at ages 9-10 and 11-12. About 370 children were exposed to cannabis before the mother knew she was pregnant, and 195 were exposed both before and after knowing the pregnancy.

The researchers looked at several neuroimaging measures that are important for brain development, including measurements of brain thickness and surface area, as well as measurements of water diffusion in and out of cells. The patterns found in the group of children exposed to cannabis before birth are consistent with possible reductions in neuroinflammation.

“It is possible that we are seeing an anti-inflammatory effect of cannabis, leading to differences in the way the brain is pruned during neurodevelopment,” Bogdan said.

Much has been made of the anti-inflammatory effects of cannabis, but reducing inflammation isn’t always a good thing. It’s all about timing. Too much reduction of inflammation at the wrong time can affect the way the brain is pruned and primed.

Another theory is that cannabis exposure leads to accelerated aging. But don’t expect to find irrefutable biological evidence linking mental health problems to early cannabis exposure.

It might not even be about pruning. It might not be the cannabis use itself, but rather the post-combustion products of smoking cannabis that can cause accelerated aging and subsequent cognitive effects, Bogdan said.

Or it all comes down to sociological factors.

Finding the one-to-one link that proves prenatal cannabis exposure has negative effects during the teenage years is challenging and may not be possible with retrospective studies. Baranger notes that the biggest limitation of this data set is that it was retrospective; mothers reported their cannabis use 10 years ago, so he looks forward to new data from prospective, longitudinal studies that will provide more recent, accurate, and detailed information about cannabis use during pregnancy.

“That may give us more answers to these questions in the future.”

In the meantime, the results of this study confirm that if you’re considering using cannabis during pregnancy, you “should talk to your doctor about your choices and what other options are available,” Baranger said.