luteal phase, the third phase of the human menstrual cycle, when the ovarian follicle that has recently discharged an ovum ruptures and transforms into the corpus luteum, which secretes progesterone. Progesterone acts on the endometrium to build up tissue with a supply of blood for nourishment of the potential embryo. If fertilization and conception do not take place, the estrogen level falls and the menstrual phase begins.
