This study looked at the expression of cannabinoid receptors in the human uterine smooth muscle during pregnancy and evaluated the effects of endogenous and exogenous cannabinoids on myometrial contractility in vitro. Human myometrial biopsy specimens were obtained at elective caesarean delivery and snap frozen for isometric recording under physiologic conditions. The results showed that both endogenous and exogenous cannabinoids exert a potent and direct relaxant effect on human pregnant myometrium, which is mediated through the CB1 receptor. This highlights a possible role for endogenous cannabinoids during human pregnancy and shows that the use of cannabis during pregnancy is not linked with pre-term labour.