Menstruation is the periodic shedding and bleeding of the endometrium with the cyclical changes of the ovaries. In each menstrual cycle, only a few follicles develop and mature, and only one of them is ovulated. The first day of bleeding is the beginning of the menstrual cycle, and the time between the first day of two menstrual periods is called a menstrual cycle, usually 28-30 days, the length of the menstrual cycle varies from person to person, but each woman’s menstrual cycle has its own pattern. As far as the cyclical changes of the ovaries are concerned, before ovulation is the follicular phase, which is the process of follicle development and maturation, and then the process of ovulation in which the egg cell is expelled together with some cells around it, and after ovulation, the luteal phase, which is the process of formation and degeneration of the corpus luteum of the ovary. If the egg is not fertilized, the corpus luteum of the ovary starts to degenerate 9-10 days after ovulation. The normal ovulation cycle is limited to 14 days of corpus luteum function, and after the luteum declines, menstruation comes and a new follicle develops in the ovary, starting a new cycle. Therefore, ovulation mostly occurs about 14 days before the next menstrual flow. Since the duration of the luteal phase is relatively fixed and the length of the follicular phase varies, when calculating the date of ovulation, the 14 days before the next menstrual period is often taken as the day of ovulation.