Cannabis use during pregnancy linked to impaired fetal growth

Cannabidiol (CBD), the component in cannabis often used for therapeutic treatments, is increasingly being used during pregnancy as a means of managing symptoms such as nausea, anxiety and sleep. Though the public perception is that CBD – particularly when consumed orally – is safer and helpful for symptom management, little is known about the impact of CBD on pregnancy.

A new study from McMaster University researchers fills that gap in knowledge and has uncovered potential risks, including impaired fetal growth, associated with the use of both tetrahydrocannabinol (THC), the primary psychoactive component of cannabis, as well as CBD, during pregnancy.

The research, published in eBioMedicine, part of The Lancet Discovery Science, on Feb. 05, 2025, found oral cannabis use from early to mid-pregnancy was associated with impaired fetal growth and changes in the placenta, as well as changes in aggression, activity levels and learning ability in offspring.

“The use of cannabis during pregnancy is rising in Canada. Most previous studies focus on exposure to cannabis smoke or just the psychoactive component of cannabis (THC), leaving other cannabis ingredients and methods of consumption understudied. However, consuming cannabis orally or only using other cannabis ingredients like CBD, is often perceived as safer despite this lack of evidence. Our study is shedding light that there’s a possibility that CBD might also be harmful in the pregnancy,” says Tyrah Ritchie, who led the study while completing her master’s degree in the Medical Sciences program at McMaster University and with the McMaster Immunology Research Centre.

Cannabis use may impact growth and behavior

The group of scientists led by Ritchie and Ali Ashkar, professor with McMaster’s Department of Medicine, conducted the research by administering cannabis oil to mouse models and followed the development of the fetuses during pregnancy and post-birth.

The researchers found that both THC and CBD impaired fetal growth causing fetuses to be small when compared to mice that did not receive cannabis. Additionally, they found that both THC and CBD impact development of the blood vessels that supply the placenta and therefore the baby with nutrients.

“We did see that both THC and CBD can disrupt a special type of immune cell called a Uterine NK Cell that is crucial in normal development of the placenta. We also saw poor development of the arteries that help supply blood to the placenta. We’re thinking this is what is possibly disrupting the nutrient and oxygen delivery to the baby and contributing to the intrauterine grow restriction,” says Ashkar.

When observing the behavior of the babies post-birth, researchers found changes in their aggression, activity levels and even learning ability.

Oral use of THC and CBD during the pregnancy not only disrupts growth of the baby but seems to have long-term consequences as the babies were found to have changes in their behavior later in life.”

Ali Ashkar, Professor, McMaster’s Department of Medicine

Ritchie says more research is needed to better understand the impacts of cannabis in humans during pregnancy, but says this study shows the possibility of it being dangerous or harmful.

Funding for the study was provided by The Michael G. DeGroote Centre for Medicinal Cancer Research, the Canadian Institutes of Health Research, and the Canadian Foundation for Innovation.