A new study from researchers at Queen’s University and Western University is the first to definitively show that regular exposure to THC, the main psychoactive ingredient in cannabis, during pregnancy has significant impact on placental and fetal development. With more than a year since the legalization of recreational cannabis in Canada, the effects of its use during pregnancy are only now beginning to be understood.

The study, published today in Scientific Reports, uses a rat model and human placental cells to show that maternal exposure to THC during pregnancy has a measurable impact on both the development of the organs of the fetus and the gene expression that is essential to placental function.

“Marijuana has been legalized in Canada and in many states in the US, however, its use during pregnancy has not been well studied up until this point," says Queen's University associate professor David Natale (Obstetrics and Gynaecology). "This study is important to support clinicians in communicating the very real risks associated with cannabis use during pregnancy."

The researchers demonstrated in a rat model that regular exposure to a low-dose of THC that mimics daily use of cannabis during pregnancy led to a reduction in birth weight of eight per cent and decreased brain and liver growth by more than 20 per cent. The research team was also able to characterize how THC prevents oxygen and nutrients from crossing the placenta into the developing fetus.

"This data supports clinical studies that suggest cannabis use during pregnancy it is associated with low birth weight babies," says Western University associate professor Dan Hardy. "Clinical data is complicated because it is confounded by other factors such as socioeconomic status. This is the first study to definitively support the fact that THC alone has a direct impact on placental and fetal growth.”

The researchers point out that there are currently no clear guidelines from Health Canada on the use of cannabis in pregnancy and some studies have shown that up to one in five women are using cannabis during pregnancy to prevent morning sickness, for anxiety or for social reasons.