1.Definition: Menstruation refers to the periodic shedding and bleeding of the endometrium that accompanies the cyclical changes of the ovaries. 2.Significance: The establishment of regular menstruation is an important sign of maturity of reproductive function. 3. Age of menarche: The age of menarche (i.e., the first menstrual flow) is mostly between 13 and 15 years old, and can be as early as 11 to 12 years old or as late as 15 to 16 years old. The age of menarche is related to nutrition, genetics, physical condition and other factors. In recent years, there is a tendency to advance the age of menarche. 4.Menstrual cycle: The first day of bleeding is the beginning of the menstrual cycle, and the interval between the first day of two menstrual periods is called a menstrual cycle. Generally, it is 21 to 35 days, with an average of 28 days. 5.Menstrual period: The duration of each menstruation is called menstrual period. Generally 2 to 8 days, an average of 4 to 6 days. 6.Menstrual volume: The menstrual volume is the total blood loss of one menstrual period, and normal menstruation is mostly 20-60ml. 7.Menstrual blood characteristics: dark red in color, containing blood, endometrial debris, cervical mucus and shed vaginal epithelial cells. The menstrual blood is not clotted, but may also form clots if the bleeding rate is too fast. 8.Concomitant symptoms: generally no special symptoms, some women may have lower abdominal and lumbosacral discomfort, a few women may have headache and mild neurological instability symptoms.