Menstrual fluid tends to be brightest red when the flow is heaviest. As the menstrual flow slows down, it takes longer to exit the vagina, as a result, the blood component loses its oxygenated red hue and becomes darker red, then brown and at times practically black.
Arterial and venous blood, proteolytic enzymes, remnants of endometrial glands, stromal cells (connective tissue), leukocytes and red blood cells are present in menstrual fluid.
Many women have dark brown (slow menstrual blood) before their period and some have the darker light flow as the period tapers off. The color of menstrual blood can vary month to month. A darker fluid most often reflects that menstrual fluid is slowly leaving the body.