What do we really know about pregnancy recognition in the horse: Is eCG the same as hCG?

The recognition of a developing embryo by the maternal system is a critical stage in the reproductive process of mammals. There is considerable diversity in the mechanism utilized by various species. In humans and other primates the developing embryo a hormone called chorionic gonadotrophin (hCG). This hormone is very similar to luteinizing hormone (LH) and binds to LH receptors in the corpus luteum and maintains the secretion of progesterone the hormone critical to the establishment and maintenance of pregnancy. In sheep and other ruminants the corpus luteum regresses at the end of the non pregnancy period in response to the rising concentrations of prostaglandin F₂α (PFG₂α) secreted by the endometrium in response to prolonged progesterone stimulation. The developing embryo in these species secretes the cytokine interferon tau which inhibits the secretion of PGF₂α by the endometrium. Horses are another species in which a chorionic hormone (eCG) is secreted by the embryo and it has largely been assumed that its role is similar to hCG.