Aunt usually stands in for menstruation. Dark brown menstrual blood may be caused by oxidization of normal menstrual blood or by irregular menstruation or structural abnormalities of the uterus. 1. Physiological reasons On the first day of menstruation, if a woman’s menstrual flow is relatively low and not discharged in time, the menstrual blood may be oxidized in the vagina and appear dark brown. 2. Pathological causes (1) Irregular menstruation: Women with dark brown menstrual blood, or less menstrual blood, irregular menstrual cycle, etc. may be due to women’s mental stress or other reasons that lead to menstrual disorders, which cause changes in menstrual traits. (2) Abnormal uterine structure: dark brown menstrual blood may be caused by abnormal uterine structure, which is too anteriorly inclined or anteriorly flexed, resulting in oxidization of menstrual blood that cannot be discharged in time and stays for a long time in the vagina. There may also be other reasons why a woman’s menstrual blood is dark brown. If the menstrual blood is dark brown and accompanied by other symptoms, it is recommended to go to the hospital in time to clarify the cause of the disease and carry out targeted treatment.