Menstruation of 3-7 days is normal, less than 3 days is called short period, more than 7 days is called prolonged menstruation. The so-called menstruation refers to the cyclical peeling and bleeding of the uterine lining that occurs along with the cyclical changes in ovarian hormones, and has its own regularity. In general, the menstrual cycle is 28 days up and down, and the week before and after is within the normal range, that is, 21-35 days are normal, less than 21 days is called frequent menstruation, and more than 35 days is called scanty menstruation. Menstruation has its regularity every month, the normal value of menstrual flow is 5-80ml, less than 5ml is called a decrease in menstrual flow, more than 80ml is called an increase in menstrual flow.