This news item is about: In horses and their close relatives (donkeys and zebras), unusual cells of the placenta invade the mother’s womb during early pregnancy. Called endometrial cups for their concave shape, they behave much like cells from metastatic tumors, leaving the placenta and migrating into the uterus, where they secrete copious amounts of the well-known pregnancy hormone, equine Chorionic Gonadotrophin (eCG).