There are many reasons why menstrual flow is low and dark brown, which may be related to endocrine function abnormality, ovarian failure and endometrial damage disease. 1. Abnormal endocrine function: This cause usually includes hypothyroidism, pituitary microadenoma and other diseases, which leads to hormone level disorders in the body, causing the endometrium to proliferate and shed abnormally, which may result in low menstrual flow and dark brown color. 2. Ovarian failure: Ovarian failure may cause low menstrual flow and dark brown color, the disease will make the ovary reserve follicle depletion, so that the ovary loses normal ovulation and cyclic hormone secretion function, estrogen level changes. 3. Endometrial injurious disease: If you have recently undergone surgery, it may cause the organism to be infected and the endometrium to be damaged, which in turn may cause the cycle change of physiological proliferation and shedding of the endometrium to disappear, resulting in a decrease in the amount of menstrual blood and a change in the color of the menstrual blood. There may also be other reasons for the dark brown menstrual flow, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, under the guidance of a professional doctor to give targeted treatment or treatment.