The normal menstrual cycle is characterized by a sequence of events that lead to menstruation. In the follicular phase, selection and maturation of the dominant follicle take place. The growing follicle produces estradiol, while following ovulation, the corpus luteum produces progesterone. Ovarian steroidogenesis takes place in the theca and the granulosa cells of the follicle with the cooperative action of LH and FSH. The sequence of endocrine events during the normal menstrual cycle depends on the activity of the ovarian feedback mechanisms. The negative feedback is mediated in the follicular phase by estradiol and in the luteal phase by progesterone, keeping gonadotropin levels low in both phases of the cycle. With the positive feedback mechanism, the midcycle LH surge is triggered and is responsible for final oocyte maturation, ovulation, and formation of the corpus luteum. Estradiol and progesterone acting οn the endometrium cause cyclic structural and morphological changes, especially in the secretory phase, with the aim of completing receptivity for embryo implantation. Otherwise, luteolysis occurs, leading to endometrial breakdown and shedding during menstruation.