Dark red period blood is likely not a cause for concern and simply indicates that a woman is nearing the end of her menstrual cycle, and her blood is leaving her body more slowly, according to WebMD. Dark red blood indicates that the uterine lining is old and less oxygenated.
Dark red period blood is common among women who have given birth and have an expanded uterus, as more blood is able to collect within the uterus and change colors, explains WebMD. Clotting may accompany dark red period blood as well as heavy menstrual cycles. If this is a common occurrence for a woman, she is encouraged to visit a doctor to have blood tests. It is possible for women who experience consistently heavy periods to suffer from enough blood loss each month that they slowly develop anemia.
Blood color changes may indicate an underlying health condition, such as hormonal imbalances, uterine fibroids or thyroid complications, according to WebMD. Benign polyps can grow within the uterus and block the normal flow of period blood from the uterus, causing blood to collect and change color. To rule out the presence of any physical abnormalities, a doctor may choose to administer imaging tests such as vaginal ultrasounds or magnetic resonance imaging scans to gain visibility into the condition of a woman's uterus, cervix and ovaries.