Marijuana linked to chromosomal defects in human egg cells, study finds
Marijuana may damage oocytes — unfertilized eggs in female ovaries — in ways researchers worry may lead to infertility, miscarriage and possible genetic defects in babies, new research has found.
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Higher levels of tetrahydrocannabinol, or THC — a chemical in marijuana that produces a euphoric feeling or "high" — were linked with changes in how eggs mature and a possible increase in chromosome malformations, according to the study published Tuesday in the journal .
"The findings of this study are concerning and highlight the importance of a cautious approach in using cannabis when planning to conceive," said obstetrician Dr. Jamie Lo, an associate professor of obstetrics, gynecology and urology in the School of Medicine at Oregon Health & Science University in Portland, in an email. She was not involved with the study.
Use of marijuana during pregnancy is rising. A found that past-month self-reported cannabis use has more than tripled among pregnant women in the United States from 2002 to 2020. Numerous studies have linked marijuana use during pregnancy to , poor fetal development, low infant , dangerously and .
The new study is "an exciting and novel approach to address a key gap in knowledge," said Lo, who studies marijuana and fertility. A number of studies have looked at how , but until now, no study had looked at the impact of marijuana on the human egg and ovarian follicle, she said.
A follicle is a small, fluid-filled sac in the ovary that contains one unfertilized egg. During a woman's menstrual cycle, hormones kick off a chain of events that help the oocyte segregate chromosomes so it will be ready for fertilization by sperm.
Higher levels of THC appeared to affect this process, said lead study author Cyntia Duval, who conducted the study while a postdoctoral research fellow in ovarian biology at the University of Toronto.
However, the study can only show an association and not a direct cause and effect, said Duval, who now directs applied research at CReATe Fertility Centre, a fertility clinic in Toronto. (CReATe stands for Canadian Reproductive Assisted Technology.)
"This is a hypothesis, and the worst thing I would want is for the public to read this and become fearful," she said. "More studies are needed to verify our findings and determine how or even if the changes we observed affect reproduction."
Higher levels of THC most harmful
Researchers analyzed over 1,000 ovarian fluid samples from patients undergoing infertility treatment. Oocytes from a smaller group of 62 patients who tested positive for THC were compared with a matched control group with no detectable use of cannabis.
Compared with the control group, the group of patients with detectable levels of THC was associated with an increased oocyte maturation rate and a lower number of embryos with the correct number of chromosomes, the study team found.
On first glance, a higher maturation rate may appear to be positive, but developing too quickly can create problems with how oocytes prepare for reproduction, Duval said.
"Chromosomes need time to align perfectly to be ready to be fertilized by sperm and create healthy embryos," she said. "So yes, we have more oocytes that are mature, but at what price if they don't have the right number of chromosomes?"
Another part of the study examined extremely immature oocytes in vitro, which means in a test tube in the lab. The oocytes were donated by women undergoing fertility treatment and were not suitable for use, Duval said.
When exposed to THC for 24 hours in a petri dish, immature oocytes receiving higher levels of THC had more altered spindles — the structures that help chromosomes to segregate properly — which can lead to failures in embryonic development.
"The finding that exposure to higher levels of THC could be associated with worsened fertility and increased reproductive complications may help patients that cannot abstain from cannabis with a harm-reduction strategy," Lo said.
"With this information, these patients can consider reducing the amount of cannabis that they are using to mitigate adverse outcomes to their babies."
The potency of THC is not always obvious
The potency of THC quadrupled between 1995 and 2022 and is now even higher, according to the National Institute on Drug Abuse.
"Cannabis flower and concentrates in dispensaries can have THC concentrations of more than 40%. Higher THC concentrations have been associated with a greater likelihood of cannabis use progressing to cannabis use disorder, among other health concerns," the Institute states on its .
This dangerous rise in potency may not be clear to consumers, experts fear. Looking at a product label might tell a person the "precise potency" of THC in a store where marijuana is legally sold. But "people buying cannabis illegally may not be able to access reliable information about the potency of the product they are using," researcher Tom Freeman, a professor of psychology at the University of Bath, told CNN in a He was not involved in the study.
"However, certain types of cannabis are typically more potent than others — cannabis extracts are typically more potent than cannabis flower," said Freeman, director of the Addiction and Mental Health Group at the university in the United Kingdom.
While people do try to adjust their consumption when the potency of their cannabis varies, "such as by adding less cannabis to their joint or inhaling less deeply," these efforts fail to completely work, he added.
That means "higher potency products still deliver a larger dose of THC to consumers than lower potency products," Freeman said.
Women may unknowingly harm the success of their future pregnancy by being exposed to higher THC levels than they intended, Lo 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."