menstrual cycle
a modified estrous cycle that occurs in most primates, including humans (in which it averages 28 days). The events of the cycle are dependent on cyclical changes in the concentrations of gonadotropins secreted by the anterior pituitary gland, under the control of gonadotropin-releasing hormone, and can be divided into two phases. In the follicular phase, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) stimulate development of an ovum and secretion of estrogen within the ovary, in a graafian follicle, culminating in ovulation, which occurs halfway through the cycle. The estrogen stimulates thickening of the endometrium of the uterus in preparation to receive a fertilized ovum. The luteal phase begins immediately after ovulation, when the ruptured follicle becomes the corpus luteum and secretes progesterone, which inhibits further secretion of releasing hormone (and hence of FSH
and LH). If fertilization does not occur, this phase ends with menstruation and a repeat of the follicular phase.