Menstruation is a normal physiological phenomenon, the cycle is regular, and the cycle is not less than 25 days, if less than 25 days is an abnormal phenomenon. 12 days early may be due to the following causes: 1. 12 days early menstruation due to luteal insufficiency; 2. 12 days early menstruation due to submucosal fibroids or large interstitial fibroids, which affect the contraction of the uterus and cause the opening of the blood sinus, often accompanied by excessive menstruation; 3. 12 days early menstruation due to taking certain drugs, such as progesterone or birth control pills. Premature menstruation may occur. It is recommended to combine the clinical history with a comprehensive analysis, and if necessary, ultrasound examination can be done to exclude organic lesions in the pelvis, and endocrine tests are also recommended to exclude the possibility of endocrine disorders.