Early menstruation of twelve days is common in the following causes. The first is luteal insufficiency due to endocrine disorders, where early atrophy of the corpus luteum occurs. Secondly, shortening of the menstrual cycle due to submucous fibroids, which is often characterized by heavy menstrual flow and irregular vaginal bleeding. Third, endometrial polyps or inflammation of the uterine lining can also lead to early menstruation by twelve days. Fourthly, tumors in the pelvis can also lead to disorders in the menstrual cycle and early menstruation. Fifth, systemic diseases such as coagulation disorders, thrombocytopenia, aplastic anemia, can lead to early menstruation, so be sure to go to the hospital for a detailed examination.