Dark brown blood before menstruation may be caused by endocrine disorders, insufficient function of the corpus luteum, endometriosis or other reasons. 1. Endocrine disorders: it means that certain endocrine glands or cells in the body cannot normally secrete hormones due to certain reasons, which will lead to dark brown blood before menstruation. 2. Inadequate function of the corpus luteum: During the menstrual cycle, there is follicular development and ovulation, but insufficient secretion of progesterone during the luteal phase or premature decline of the corpus luteum, resulting in poor secretion of the endometrium, which will lead to dark brown blood before menstruation. 3. Endometriosis: endometrial tissue with growth function appears in the uterine cavity and parts of the uterus other than the myometrium, which can also lead to dark brown blood before menstruation. There may also be other reasons for dark brown blood before menstruation. It is recommended to go to the hospital in time, complete the examination to clarify the cause of the disease, and then follow the doctor’s instructions to carry out targeted treatment or therapy.