Irregular menstruation in twelve-year-old girls entering puberty is generally normal. However, endocrine disorders (such as polycystic ovary syndrome, etc.), organic lesions of the reproductive organs (such as uterine fibroids, etc.), inflammation (such as vaginitis, etc.) and other abnormal phenomena may also occur. 1. Normal phenomenon: Menstruation is generally the cyclic shedding of the endometrium under the action of estrogen, which manifests itself as cyclic blood discharge and so on. Due to the unstable secretion of estrogen at the time of menarche, the follicles can not be discharged normally, so the phenomenon of irregular menstruation will occur, and will gradually return to normal in one or two years. 2. Irregular phenomena: (1) Polycystic ovary syndrome: girls may have abnormal menstrual cycles, low menstrual flow or amenorrhea, accompanied by symptoms of excessive androgen secretion such as acne or sudden luxuriant body hair. (2) Uterine fibroids: sudden increase in menstrual flow, prolonged menstrual period or non-menstrual bleeding accompanied by symptoms such as frequent urination, urgency and constipation. (3) Vaginitis: there may be an increase in the amount of leukorrhea before and after menstruation, odor and other leukorrhea abnormalities or self-conscious itching, vulvar burning and other symptoms. In addition, there are many other reasons why twelve-year-old girls have irregular menstruation, and it is recommended that patients seek timely medical attention to identify the cause of the disease and then carry out treatment as soon as possible.