The monthly cycle of changes in the ovaries and the lining of the uterus (endometrium), starting with the preparation of an egg for fertilization. When the follicle of the prepared egg in the ovary breaks, it is released for fertilization and ovulation occurs. Unless pregnancy occurs, the cycle ends with the shedding of part of the endometrium, which is menstruation. Although it is actually the end of the physical cycle, the first day of menstrual bleeding is designated as "day 1" of the menstrual cycle in medical parlance.