The luteal phase (or secretory phase) is the latter phase of the menstrual cycle (in humans and a few other animals) or the estrous cycle (in other placental mammals). It begins with the formation of the corpus luteum and ends in either pregnancy or luteolysis. The main hormone associated with this stage is progesterone, which is significantly higher during the luteal phase than other phases of the cycle. Some sources define the end of the luteal phase to be a distinct "ischemic phase".