There are several cases of brown blood on the first day of menstruation: 1. It may be because of gynecological inflammation, such as common vaginitis, endometritis, pelvic inflammatory disease, etc., especially vaginitis can easily cause a change in the color of menstruation. This is because when there is inflammation the inflammatory exudate material as well as the inflammation, the necrosis caused will be discharged with the menstrual blood, thus causing the color of the menstrual blood to change, which can make the color of the menstrual blood brown; 2. It may be because the uterus is suffering from other diseases, such as endometrial cancer, cervical cancer, mucinous cancer of the uterus and other kinds of cancer or sarcoma. This is because when the uterus suffers from malignant diseases, the abnormally proliferating substances themselves are prone to necrosis, thus forming erosions and ulcers that can be secondary to infection, and these abnormally proliferating substances themselves can have secretions formed. Because normal menstruation is formed by the periodic contraction of the small arteries of the endometrium, when the uterus suffers from these diseases it can cause the menstrual blood to appear brown.