A 15-year-old girl’s menstruation lasts for more than 20 days may be caused by the imperfect development of the endocrine axis or endocrine hormone disorders during puberty. 1. Imperfect development of the endocrine axis: 15-year-old girls menstruation lasts for more than 20 days may be because the uterus and the hypothalamus hormone axis is not well developed during puberty, so that women appear to have a long period of menstruation, and 15-year-old girls may appear to have menstruation last for more than 20 days and still do not end the situation. 2. Endocrine disorders: 15-year-old girl menstruation lasts for more than 20 days may also be due to the high pressure of study, mental long-term high tension, so that the endocrine system disorders of menstrual irregularities. Based on the 15-year-old girl menstruation lasts more than 20 days can not accurately determine the cause, but also need to be combined with the characteristics of the medical history, other test results in order to make an accurate judgment, it is recommended that under the guidance of the doctor to clarify the cause of the disease, treatment, to prevent the emergence of anemia and other conditions.