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Recent Studies on Effects of Cannabis in the Prenatal Brain

Cannabis is the most commonly used illicit drug during pregnancy.

Key points

  • Some females of child-bearing age do not perceive cannabis to be dangerous.
  • Prenatal cannabis exposure is associated with impairments in physical and neurological development during infancy.
  • The active ingredients in cannabis can easily cross into the placenta and act on fetal brain.
  • Prenatal cannabis produces stronger hippocampal connections to the frontal cortex but reduces connectivity in other brain regions.

Cannabis is the most commonly used illicit drug during pregnancy. There are many reasons. Many young women are convinced by the myths on the internet that the ingredients of cannabis offer therapeutic benefits for anxiety disorders (barely true), epileptic encephalopathy (true for THC, but not for CBD), psychiatric disorders (no evidence for this), and cancer (definitely not true). Many women use cannabis prior to knowing they are pregnant and then continue use, hoping to relieve their morning sickness, depression, stress, and anxiety.

Many of my female students of child-bearer age do not perceive cannabis to be dangerous. Cannabis-using pregnant women often meet the criteria for having cannabis use disorders relative as compared to cannabis-using non-pregnant women of a similar reproductive age. This is worrisome because it suggests that pregnant women use more cannabis than non-pregnant women of the same age group.

It is well known that prenatal cannabis exposure is associated with impairments in physical and neurological development during infancy, including decreased birthweight and longer stays in neonatal intensive care units. Later in life, fetal cannabis exposure is associated with diminished visual perception, language comprehension, impulse control, sleep disturbances, and attention among school-age children.

Why does cannabis so easily affect the fetal brain?

The active ingredients in cannabis can easily cross into the placenta and act on the fetal brain where cannabinoid receptors are already waiting to respond. Cannabinoid receptors are among the most common receptors in the fetal brain. The overstimulation of endogenous cannabinoid receptors dramatically influences cellular differentiation, axonal growth, and the migration of cells. The type-1 cannabinoid receptors play a critical role in the ability of axons to find their proper paths through the brain to make the correct connections with other neurons.

The migration of neurons within the hippocampus is particularly vulnerable to the presence of cannabis. One reason is that many of these neurons express cannabinoid receptors. In addition, research in my laboratory has shown that cannabinoid receptors are expressed by stem cells in the hippocampus that give rise to new neurons that are critical for development. Cannabinoids impact brain development from the level of genes to complex brain systems that can predispose a child to the development of psychiatric disorders later in life.

The present study examined the functional connectivity of the hippocampal network. The current study included data from 115 pregnant, second- and third-trimester mothers. The MRI studies were conducted when the fetuses were between 22 and 39 weeks gestational age. Fetuses exposed to prenatal cannabis demonstrated stronger hippocampal connections to the frontal cortex but reduced hippocampal connectivity to brain regions in the back half of the brain, such as the parietal lobe.

What do these results indicate?

No one is sure, yet. Scientists still have only a limited understanding of associations between variations in the prenatal brain network and its consequences on future neurobehavioral health. The results of the current study indicate that by artificially, and repeatedly, stimulating cannabinoid receptors in the fetal brain that the growth and pattern of neural connections are significantly altered. Many recent studies have demonstrated that alterations in fetal neural connections contribute to neurobehavioral vulnerability later in life, including predisposition to psychiatric illness or mood disorders.

References

Thomason ME et al (2021) Miswiring the brain: Human prenatal Delta 9-tetrahydrocannabinol use associated with altered fetal hippocampal brain network connectivity. DEVELOPMENTAL COGNITIVE NEUROSCIENCE 51, early access.

Marchalant, Y., Brothers, H.M. & Wenk, G.L. (2009) Cannabinoid agonist WIN-55,212-2 partially restores neurogenesis in the aged rat brain. Molecular Psychiatry, 14:1068-1071. PMCID: PMC3011092

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