Smoking cannabis could have a catastrophic effect on fertility and fetal development, a new study suggests. 

In the US, cannabis consumption has almost doubled over the past ten years, with 21.8 percent of people aged 12 and older – approximately 61.8 million individuals – reporting marijuana use in 2023, an increase from 12.6 percent in 2013.

Researchers from the University of Toronto in Canada suggest that this significant surge, driven partly by the decriminalization and legalization of recreational cannabis, has heightened concerns regarding the drug’s effects on fertility.

To delve deeper into this issue, they conducted two studies. The first study was a laboratory experiment where oocytes, or immature human eggs, were exposed to THC – the psychoactive component responsible for the ‘high’ in marijuana – to examine their chromosomal development.

In the second study, the team analyzed data from 1,059 egg cell fluid samples obtained from patients undergoing in vitro fertilization (IVF), a fertility treatment involving the creation of embryos in a lab from eggs and sperm for implantation.

Out of these samples, sixty-two tested positive for THC, which represents a six percent positivity rate. The researchers then compared these to the ‘clean’ samples to evaluate variations in the development of the eggs’ cells.

In both studies, the researchers discovered an association between THC and reduced rates of embryos with the correct number of chromosomes, which can lead to birth defects and miscarriages.

In the analysis of IVF patients, 60 percent of embryos in the THC-positive group had the proper number of chromosomes, compared to 67 percent in the THC-negative group. 

Smoking cannabis could have a catastrophic effect on fertility and fetal development, a new study suggests (stock image)

In the lab study, THC exposure led to a nine percent increase in rates of embryos containing an improper number of chromosomes, and more than doubled the rate of abnormalities in spindles, structures that assist in chromosomal development. 

However, the researchers caveated their results, noting that people undergoing IVF are taking large amounts of hormones, so their findings may not be generalizable to the larger public.  

Samples used in the study came from females under the age of 40 and it is unclear as to when they consumed THC products.

Most chromosomally abnormal embryos will either fail to implant in the uterus or will result in miscarriage. 

In more rare instances, the abnormal embryos may result in chromosomal disorders, such as Down syndrome.

Additionally, exposing immature egg cells to THC disrupted their ability to sort chromosomes.

An abnormality in the way an egg cell’s chromosomes line up during cell division is linked with miscarriage and birth defects such as Edwards syndrome, which causes deadly growth delays, and Patau syndrome, which causes physical and developmental abnormalities.

The Autism Research Institute notes that previous research has also found that prenatal exposure to cannabis may increase likelihood of autism, with animal studies suggesting THC can lead to genetic mutations. 

Similarly, chromosomal abnormalities and defects are tied to autism.

These effects were seen in the recent study with people who had THC in their systems from marijuana products while they were pregnant or undergoing IVF.  

Off the back of their investigation, the authors of the recent study conclude: ‘These findings underscore the need for increased awareness and caution among people with ovaries, particularly those undergoing fertility treatments. 

‘Our study highlights the importance of informing patients about the potential risks associated with cannabis consumption and provides a basis for regulatory bodies, medical professional societies, and public health organizations to establish recommendations and guidelines regarding cannabis consumption during fertility treatment.’ 

The findings were published in the medical journal, Nature Communications.  

In the analysis of IVF patients, just 60 percent of embryos in the THC-positive group had the proper number of chromosomes, compared to 67 percent in the THC-negative group. These ‘abnormal’ results are pictured above

Previous research has also looked at how marijuana can harm fertility. 

A 2023 study led by the University of California, Irvine (UCI), found adolescent female mice exposed to tetrahydrocannabinol (THC), the psychoactive chemical in the drug, had 50 percent fewer healthy ovarian follicles than the control group by the time they were adults.

Researchers believe the body’s endocannabinoid system – which is stimulated when marijuana is used – damages follicles or causes them to activate too quickly, depleting them.

With marijuana use now commonplace among teens across America, researchers fear many young women are unknowingly damaging their likelihood of having a child in the future.

While the study was on mice, separate research has shown that pregnant women who use cannabis are at a greater risk of birth defects, miscarriages and stillbirths.

Other research has found that marijuana may damage semen quality and affect male fertility too

In a 2018 study out of Tulane and the University of Washington, researchers looked at samples from men in Washington, where the drug has been legal for recreational use since 2012, and compared it with their cannabis consumption.

They found that men who had consumed cannabis in their lifetime were more likely to have abnormally-shaped sperm and lower concentrations of sperm compared to men who had never used the drug.