Menstruation is the periodic shedding and bleeding of the endometrium in response to the cyclical changes of the ovaries. During each menstrual cycle, only a few follicles develop and mature, and ovulation occurs in only one of them. The process of ovulation occurs when the oocyte is expelled along with some of its surrounding cells. Before ovulation, the follicle enters a preovulatory state, with the oocyte and the radial crown floating in the follicular fluid and the follicle protruding from the ovarian surface like a blister. Ovulation usually occurs about 14 days before the next menstrual period, and the eggs can be discharged from both ovaries in turn or continuously from one ovary. After ovulation, the egg enters the fallopian tube and runs towards the uterus through the umbrella of the fallopian tube, peristalsis of the fallopian tube wall and the ciliary activity of the mucous membrane of the fallopian tube. It is clear from this that ovulation is a momentary event, but 2-3 days before and after the day of ovulation is the range of the ovulation period, during which it is easy to conceive and is called the fertile period. It is not only on the day of ovulation that conception occurs, because the egg usually survives for 2-5 days after ovulation, while the sperm can survive in the female reproductive tract for 1-3 days, so the ovulation period is an approximate range.