When During Pregnancy THC Exposure Happens May Determine the Brain Effects
In a rat model, the timing of THC exposure during pregnancy produced different effects on the developing amygdala's immune cells and social behavior—suggesting when matters as much as whether.
Quick Facts
What This Study Found
Cannabis is the most commonly used illicit drug during pregnancy, with rates rising as legalization expands. Many pregnant users consider it less harmful than pharmaceuticals or alcohol. This study tested a critical question: does the timing of THC exposure during development change the outcome?
Using a rat model designed to approximate different windows of human pregnancy, the researchers exposed developing animals to THC at either earlier or later gestational timepoints and examined effects on the amygdala—a brain region central to social behavior and emotion.
The focus was on microglia, the brain's resident immune cells. These cells do more than fight infection during development—they actively sculpt neural circuits by engulfing and eliminating synapses and cells through phagocytosis. The research group had previously shown that endocannabinoid signaling in the developing amygdala drives microglia to consume newborn astrocytes, a process with lasting consequences for the neural circuits that govern sex differences in social behavior.
Since microglia express both CB1 and CB2 cannabinoid receptors, they're direct targets for THC. The key finding: THC exposure at different developmental windows produced distinct outcomes on microglial behavior and social functioning, paralleling what's known about fetal alcohol syndrome (where timing of alcohol exposure determines which organ systems are affected).
Key Numbers
THC exposure at different gestational timepoints produced distinct effects on amygdala microglia and social behavior in rat offspring. Microglia express both CB1R and CB2R cannabinoid receptors.
How They Did This
Animal study (rats) modeling human gestational cannabis exposure at different developmental timepoints. Examined effects on amygdala microglia (phagocytic activity, morphology) and social behavior outcomes. Built on prior work showing endocannabinoid-mediated microglial sculpting of amygdala circuits.
Why This Research Matters
The "timing matters" finding has significant implications. If early-pregnancy THC exposure produces different brain effects than late-pregnancy exposure, broad advice to "avoid cannabis during pregnancy" may need to be refined with specific risk windows—similar to how alcohol's teratogenic effects vary by trimester. It also suggests that studies looking at prenatal cannabis exposure as a binary (yes/no) may be missing important nuance.
The Bigger Picture
This adds mechanistic depth to RTHC-00151's finding of reduced neonatal brain volume with prenatal cannabis exposure. While that human study measured brain size, this animal study identifies a specific cellular mechanism—microglial disruption in the amygdala—that could explain how THC affects brain development. The sex-difference angle (endocannabinoid signaling governs sex differences in amygdala development) also raises the possibility that prenatal THC might affect male and female offspring differently.
What This Study Doesn't Tell Us
Rat brain development doesn't perfectly map onto human gestation. THC doses in animal studies may not reflect typical human use patterns. The social behavior outcomes in rats are proxies for human social function. The prior work this builds on found sex differences, but this study's design details on sex-specific effects aren't fully described in the abstract. Translation from animal to human prenatal risk is always uncertain.
Questions This Raises
- ?Do the timing-dependent effects translate to human pregnancy trimesters?
- ?Are the microglial and social behavior changes reversible after birth?
- ?Do male and female offspring show different vulnerabilities to prenatal THC at different timepoints?
Trust & Context
- Key Stat:
- Evidence Grade:
- Preclinical animal study with a mechanistic focus—provides biological insight but can't directly predict human outcomes.
- Study Age:
- Published in 2025 in Neuropsychopharmacology, building on the group's prior endocannabinoid-development research.
- Original Title:
- Timing matters: modeling the effects of gestational cannabis exposure on social behavior and microglia in the developing amygdala.
- Published In:
- Neuropsychopharmacology : official publication of the American College of Neuropsychopharmacology, 50(11), 1655-1664 (2025) — Neuropsychopharmacology is a well-respected journal focusing on the effects of drugs on the brain and behavior.
- Authors:
- Pham, Aidan L(2), Marquardt, Ashley E(2), Montgomery, Kristen R(2), Sobota, Karina N, McCarthy, Margaret M, VanRyzin, Jonathan W
- Database ID:
- RTHC-07364
Evidence Hierarchy
Watches what happens naturally without intervening.
What do these levels mean? →Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-07364APA
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.. Neuropsychopharmacology : official publication of the American College of Neuropsychopharmacology, 50(11), 1655-1664. https://doi.org/10.1038/s41386-025-02173-5
MLA
Pham, Aidan L, et al. "Timing matters: modeling the effects of gestational cannabis exposure on social behavior and microglia in the developing amygdala.." Neuropsychopharmacology : official publication of the American College of Neuropsychopharmacology, 2025. https://doi.org/10.1038/s41386-025-02173-5
RethinkTHC
RethinkTHC Research Database. "Timing matters: modeling the effects of gestational cannabis..." RTHC-07364. Retrieved from https://rethinkthc.com/research/pham-2025-timing-matters-modeling-the
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.