It is definitely not normal to have your period 12 days early. This is because normal menstruation is called normal when the cycle is physiologically regular and there is no change in the volume or color of menstruation. The most common clinical cause of 12 days early menstruation is due to insufficient luteal function, such as premature atrophy of the corpus luteum, which causes early menstruation. It is also recommended to rule out the possibility of submucosal fibroids, endometrial polyps and endometriosis, and to rule out the possibility of early menstruation due to thrombocytopenia and abnormal blood clotting function.