Chemicals in marijuana may affect women’s fertility

When Canada legalized cannabis in 2018, its effects on human health were all over the news. Cyntia Duval, a women’s health researcher at the University of Toronto at the time, wondered how its consumption might affect female fertility. To her surprise, there was almost no information on the subject — though there was plenty of data on marijuana’s effects on pregnancy and male fertility.

Chemicals in cannabis may push eggs to become ready for fertilization. But this may come at a cost: more eggs with the wrong number of chromosomes, Duval and colleagues now report in a study published September 9 in Nature Communications.

Delta-9-tetrahydrocannabinol, or THC, is the main psychoactive chemical in marijuana. It binds to cannabinoid receptors in the brain. But these receptors are all over our bodies, including in our reproductive organs.  

The receptors usually bind endocannabinoids, molecules naturally produced by the body and essential for normal bodily functions like the production of eggs and sperm. Consuming THC can affect cannabinoid receptors in the reproductive system. Many studies report that using cannabis decreases sperm count and motility. Men are usually told to avoid cannabis for at least three months before trying to conceive, Duval says. But what about women?

That has been hard to answer. Women are born with a set number of immature eggs in their ovaries. After puberty, a single egg will mature every month, becoming ready for fertilization. One of the few ways to study women’s eggs is during in vitro fertilization, or IVF. For IVF, women receive hormones that make multiple eggs mature at once, which are then collected to create embryos.

Duval, who works at CReATe Fertility Centre, an IVF clinic in Toronto, analyzed eggs and fluid collected from 1,059 women who had IVF in the clinic from 2016 to 2023. She found THC in the fluid collected with the eggs from 62 of the women. Women with higher THC levels around their eggs had larger numbers of mature eggs.

But when Duval artificially matured eggs and exposed them to THC, she found they often had the wrong number of chromosomes, which could lead to failure to form embryos, unsuccessful uterine implantation and nonviable pregnancies.

Only a much larger study could say if this decreases women’s chances of conceiving, Duval says. But the results hint at THC’s effect on female fertility.

“We have information about the male part. We have information about pregnancy, but there was a gap,” Duval says.