If the menstrual period is three days early is clinically normal, but if it is further advanced on top of that it is a shortening of the menstrual cycle, which is an abnormal phenomenon. The causes are mainly the following: first, common endocrine disorders. For example, premature atrophy of the corpus luteum leads to insufficient luteal function, which causes early menstruation. The second, submucosal fibroids, as they grow into the uterine cavity, increase the relative size of the endometrium and affect the contraction of the uterus, resulting in a shorter menstrual cycle. Third, polyps in the endometrium or inflammation of the endometrium can also shorten the menstrual cycle. In addition, tumors of the pelvis and endometriosis may cause early menstruation. Therefore, this phenomenon should be taken seriously clinically and relevant examinations should be done, such as sex hormone test, ultrasound and gynecological examination. Based on the results of the tests, the necessary treatment should be given.