Brown menstrual blood is likely just old blood that is slowly leaving the uterus, especially if it occurs towards the end of a woman's cycle, explains WebMD. The presence of brown blood may also accompany heavy bleeding or thick clots visible in a woman's menstrual discharge.
Most women experience dark period blood that appears brown or black at some point during their lives, and the occurrence often is not a medical concern, according to WebMD. If a woman also clots and notices that it happens often, she should see a doctor to have her blood levels checked to make sure she doesn't have anemia. It is possible for women with consistently heavy periods to experience enough blood loss that their bodies are unable to replenish the blood before their next cycle begins.
In some cases, dark period blood is caused by an underlying health concern, such as an internal obstruction that blocks the flow of blood from the uterus through the cervix. Women who have given birth may develop permanently enlarged uteri that produce increased amounts of blood, which is then able to collect and change color. If a woman experiences abnormal bleeding and is concerned about the cause, she should visit a doctor or gynecologist for a pelvic exam, advises WebMD.