12-year-old girl menstruation time is too long, the most important reason is due to endocrine factors, pre-pubertal hypothalamus – pituitary – ovary axis development is not too mature, so this time is prone to anovulation caused by abnormal uterine bleeding, in this case, it is recommended that you can oral contraceptive pills or progesterone after half-cycle treatment. Menstruation refers to the cyclical stripping and bleeding of the endometrium that occurs along with the cyclical changes in ovarian hormones. Menstruation has its own regularity, and the normal menstrual cycle is about 28 days, with a delay of one week before and after being considered normal, that is, 21-35 days are considered normal. The normal menstrual period is 2-8 days, less than 2 days is called short period, more than 8 days is called long period. The normal range of menstrual flow is 20-60 ml, less than 20 ml is called menorrhagia, and more than 80 ml is called menorrhagia.