In rats, THC exposure during early versus late pregnancy produced opposite effects on brain immune cell activity and social behavior, with sex-specific patterns.
Pregnant women considering cannabis use, obstetricians, developmental neuroscientists
Early vs late pregnancy THC exposure produced opposite developmental effects
What the researchers found
Postnatal THC exposure (modeling late pregnancy) produced sex-specific changes in microglial phagocytosis during brain development and altered social behavior during the juvenile period. Prenatal THC exposure (modeling early pregnancy) produced inverse changes to both phagocytosis and social behavior compared to postnatal exposure. The effects were mediated through microglia, the brain's immune cells, which express both cannabinoid receptors.
Why it matters
Cannabis use during pregnancy is increasing, and many users assume earlier use is less harmful. This study suggests the timing of exposure during pregnancy matters enormously, with early and late exposure producing opposite developmental effects. This complicates any simple guidance about cannabis use during pregnancy.
The numbers in context
Postnatal THC: sex-specific microglial phagocytosis changes and altered juvenile social behavior. Prenatal THC: inverse changes in both phagocytosis and social behavior. Effects mediated through CB1R and CB2R expressed on microglia in the developing amygdala.
How the study worked
Animal study exposing rats to THC either prenatally (via dam injection, modeling early human pregnancy) or postnatally (via direct injection in early postnatal period, modeling late human pregnancy). Microglial phagocytosis in the amygdala and social behavior were measured.
What this study cannot tell us
Animal model using direct THC injection, which differs from human cannabis use. Cannot directly translate rat developmental periods to human trimesters. Focused on amygdala and social behavior; other brain regions and behaviors were not assessed. Preprint, not yet peer-reviewed.
How to read the evidence
Preliminary evidence from an animal study (preprint) examining timing-dependent effects of prenatal THC.
When this study was published
2025 preprint examining gestational timing of THC exposure in rats.
The bigger picture
The endocannabinoid system guides normal brain development, and disrupting it with exogenous THC can alter the trajectory of neural circuit formation. The finding that timing produces opposite effects highlights the complexity of prenatal drug exposure and explains why epidemiological studies may find inconsistent results when they don't account for trimester of exposure.
Questions still open
- Do these timing-dependent effects translate to human pregnancy? Could the opposite effects at different time points explain inconsistent results in human epidemiological studies? Are there critical windows during pregnancy where THC exposure is particularly harmful?
Common questions
Does it matter when during pregnancy cannabis is used?
How does THC affect the developing brain?
Read the original research
Timing matters: modeling the effects of gestational cannabis exposure on social behavior and microglia in the developing amygdala.
bioRxiv : the preprint server for biology
Citation
Pham, Aidan L; Marquardt, Ashley E; Montgomery, Kristen R; Sobota, Karina N; McCarthy, Margaret M; VanRyzin, Jonathan W. (2025). Timing matters: modeling the effects of gestational cannabis exposure on social behavior and microglia in the developing amygdala.. bioRxiv : the preprint server for biology. https://doi.org/10.1101/2025.02.17.638714
Explore the wider topic
- How to Talk to Your Teenager About Weed (Without Losing Them)
- My Kid's Friends Smoke Weed: A Parent's Guide
- I Smoke Weed But Don't Want My Kids To: Navigating the Hypocrisy
- Quitting Weed as a Parent: Why It Is the Hardest and Most Important Quit
- Quitting Weed as a Teen or Young Adult
- My Kid Is Smoking Weed: A Parent's Guide