There are not four reasons to be wary of. The dark color of menstruation is considered to be at the beginning or end of menstruation, or due to polycystic ovary syndrome, endometriosis, uterine adhesions and other diseases. 1. Early or late menstruation: When a woman is in the early or late menstruation period, due to the relatively small amount of menstruation, when the blood comes into contact with air oxidation, the dark color of menstruation will occur. 2. Polycystic ovary syndrome: The cause of polycystic ovary syndrome is not yet clear, and it may be due to genetic and environmental factors. Patients may have symptoms such as scanty menstruation, low menstrual flow, hairy, etc. Due to the low menstrual flow, the dark color of menstruation may appear when it meets air oxidation. 3. Endometriosis: The dark color of menstruation is also considered to be caused by endometriosis, the cause of which is related to heredity, immunocompromise, inflammation, and poor sex life. Symptoms are lower abdominal pain, prolonged menstruation, premenstrual spotting and so on. 4. Uterine adhesion: Uterine adhesion may be caused by endometrial trauma, infection and other factors. Patients will have less menstruation or even amenorrhea, which is manifested as dark-colored menstruation. There are many reasons for the dark color of menstruation, so patients are advised to go to the hospital and get a clear picture of the cause of the disease.