It is very abnormal to have a menstrual period once every 20 days, which is often caused by the following reasons: 1. The most common one is menstrual disorder due to endocrine disorder, it is recommended to have a blood test for sex hormone six on the second day of menstruation, and according to the result, normal regulation must be given. 2. The menstrual cycle is shortened due to submucosal fibroids, because submucosal fibroids grow into the uterine cavity. This causes a relative increase in the size of the uterine cavity. In addition, it also affects the contraction of the uterus, leading to an early menstrual cycle, and some people may experience anemia as a result, which clinically requires surgery.3. The shortened menstrual cycle is caused by tumors or polyps in the uterine cavity, so if the menstruation comes once every 20 days, a detailed examination must be done for ultrasound, sex hormone six, and blood tests, and then the necessary treatment will be given.