Study Links THC Exposure to Chromosomal Abnormalities in Human Eggs, Raises Fertility Concerns

Researchers at the University of Toronto reported that exposure to tetrahydrocannabinol (THC), the primary psychoactive component of cannabis, was associated with higher rates of chromosomal abnormalities in immature human egg cells and with a lower proportion of chromosomally normal embryos among in vitro fertilization (IVF) patients.

In two complementary analyses published in Nature Communications, investigators conducted a laboratory experiment exposing human oocytes to THC and examined 1,059 follicular fluid samples collected from people undergoing IVF. Sixty‑two of the IVF fluid samples, or about 6 percent, tested positive for THC. Among embryos created from the THC‑positive samples, 60 percent were chromosomally normal (euploid), compared with 67 percent in the THC‑negative group.

In the laboratory component, exposure of immature egg cells to THC was associated with a roughly 9 percent increase in embryos containing an incorrect number of chromosomes and more than doubled the rate of abnormalities in spindle structures, which are responsible for separating chromosomes during cell division. The researchers reported that THC disrupted the ability of oocytes to properly sort chromosomes during division, a process linked to miscarriages and chromosomal disorders such as Edwards syndrome and Patau syndrome.

The authors cautioned that the IVF sample may not represent the general population. Participants were under age 40 and were receiving hormone stimulation as part of fertility treatment, and the timing and quantity of THC use relative to egg retrieval were not always clear. The team noted these limitations when interpreting whether the findings extend to people not undergoing fertility treatment.

Most embryos with chromosomal abnormalities fail to implant or result in miscarriage, the paper said. In rarer instances, abnormal embryos can lead to live births with chromosomal disorders such as Down syndrome. The study’s findings prompted the authors to call for increased awareness and caution among people with ovaries, particularly those pursuing fertility treatments, and for professional societies and public health bodies to consider guidance on cannabis consumption during fertility care.

The report arrives amid a steep rise in cannabis use in the United States over the past decade. National survey data cited by the authors indicate that 21.8 percent of people aged 12 and older — about 61.8 million Americans — reported past‑year marijuana use in 2023, up from 12.6 percent in 2013. Legalization and decriminalization in several states have been credited with part of that increase.

Previous research has suggested multiple reproductive and developmental risks linked to cannabis. A 2023 animal study from the University of California, Irvine, found that female mice exposed to THC during adolescence had about 50 percent fewer healthy ovarian follicles by adulthood than unexposed controls, a change researchers attributed to effects on the endocannabinoid system. Separate human studies have found associations between cannabis use and adverse pregnancy outcomes, including higher risks of birth defects, stillbirth and miscarriage. Research on male fertility has also shown links between cannabis use and lower sperm concentration and more abnormally shaped sperm.

The Autism Research Institute and other investigators have noted prior studies reporting associations between prenatal cannabis exposure and an increased likelihood of autism spectrum disorder, with some animal studies suggesting that THC can induce genetic or developmental changes. The University of Toronto authors and other experts stressed that while chromosomal abnormalities have known connections to neurodevelopmental outcomes, causal pathways between prenatal cannabis exposure and autism in humans require further study.

Clinicians and fertility specialists interviewed in related coverage said the new findings provide additional data to discuss with patients planning pregnancy or undergoing assisted reproduction, but they also emphasized the need for more research to determine risks across different patterns of use, timing of exposure and whether effects differ between recreational and medical cannabis products.

The study’s publication adds to a growing body of literature prompting health practitioners and policymakers to weigh potential reproductive risks of cannabis as its use becomes more common. The authors recommend that reproductive health providers inform patients about the possible associations and that regulators and professional organizations consider issuing guidance for people seeking conception or fertility treatment.

Further research with larger, prospectively collected human samples and clearer exposure timing will be needed to clarify how THC exposure before conception or during early pregnancy affects chromosomal segregation, embryo development and long‑term outcomes for offspring.