High levels of THC—the compound in marijuana that causes a “high”—may affect how eggs develop and could lead to fertility problems, miscarriages, and chromosome issues in embryos, according to new research.
The findings were reported on September 9 in the journal Nature Communications.
Study Details
Researchers analyzed more than 1,000 samples of ovarian fluid from patients undergoing fertility treatment. They compared unfertilized eggs (oocytes) from 62 women who tested positive for THC with a control group who did not use cannabis.
The study found that women with detectable levels of THC had a higher egg maturation rate. However, they also produced fewer embryos with the correct number of chromosomes.
Cyntia Duval, who led the study as a postdoctoral fellow at the University of Toronto, told CNN: “Chromosomes need time to align perfectly to be ready to be fertilized by sperm and create healthy embryos. So yes, we have more oocytes that are mature, but at what price if they don’t have the right number of chromosomes?”
The immature eggs were donated by women undergoing fertility treatment and were not suitable for use, Duval said.
Laboratory Findings
When the eggs were exposed to THC in a lab for 24 hours, researchers found more altered spindles—structural issues that affect chromosomes. These changes can interfere with healthy embryo development.
However, Duval noted that the study can’t prove cause and effect.
She told CNN: “This is a hypothesis, and the worst thing I would want is for the public to read this and become fearful. More studies are needed to verify our findings and determine how or even if the changes we observed affect reproduction.”
Rising Cannabis Use in Pregnancy
The findings coincide with a rise in marijuana use during pregnancy. A June study found that past-month cannabis use among pregnant women more than tripled in the United States between 2002 and 2020.
Dr. Jamie Lo, an obstetrician and associate professor at Oregon Health & Science University in Portland, told CNN: “The findings of this study are concerning and highlight the importance of a cautious approach in using cannabis when planning to conceive.”
She added in an email that the results could support a harm-reduction strategy.
Lo explained: “With this information, these patients can consider reducing the amount of cannabis that they are using to mitigate adverse outcomes to their babies.”
Increasing THC Potency
THC potency has quadrupled between 1995 and 2022, with some cannabis concentrates reaching 40% THC or higher, according to the National Institute on Drug Abuse.
The Institute’s website states: “Higher THC concentrations have been associated with a greater likelihood of cannabis use progressing to cannabis use disorder, among other health concerns.”
Medical Advice
As such, Lo offered a cautionary note.
She said: “Because it’s hard to know the specific level of THC in various cannabis products on the shelves, I advise my patients to consider safer alternatives to treat the symptoms they’re using cannabis for or at least try to reduce the frequency of their use.”
This research adds to growing concerns about cannabis use among women of reproductive age and those trying to conceive, suggesting that THC exposure may have previously unrecognized effects on fertility and embryo development. While more research is needed to establish causation, the findings support a precautionary approach to cannabis use when planning pregnancy.
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