The onset of menstruation before the age of 10 is considered precocious, while menstruation in the fifth grade at the age of 11 is normal. Each stage of a woman’s life has different physiological characteristics, among which the most significant changes are in the reproductive system, which undergoes a series of physiological changes around the menstrual cycle. Menstruation is the periodic shedding and bleeding of the endometrium that accompanies the periodic ovulation of the ovaries, and is one of the signs of the maturity of female reproductive function. Women can determine whether or not they are pregnant through menstruation, as well as guide safe periods for contraception, etc. Menstruation shows cyclical changes with a normal interval of 21-35 days, with an average interval of 30 days, a continuous bleeding time of 3-7 days each time, and a menstrual volume of 5-80 ml.