It is not unusual for the blood flow during menstruation to change color from vivid red to blackish brown. Blood passed out during menstruation more often than not clots in the uterus and these clots break up before moving to the vagina. There are times when these clots are also known to move through the cervical opening before breaking up.
During the course of menstruation the endometrium also known as the uterine lining starts to shed and during this process you may notice dark clumps of mass- like tissue. This is altogether a normal process. Observing very dark clots in the color range of dark brown or black is not something to be frightened or concerned about.
Blood is thrust outwards from your uterus when the uterus narrows. During such time it is not necessary that you feel uterus contraction. Indications of the uterus contracting are symptoms of cramps when these contractions are really strong. Nature helps the menstruation process become easier when the uterus starts to release a type of anti-coagulants. These anti-coagulants when released in the system help to keep the menstruation blood from coagulating up as it moves from the uterus to the vagina. In many ways than one the anticoagulants cause the bleeding procedure to happen with ease.
However during days of strong cramp sensations and severe bleeding, you are likely to pass out dark, lumpy clots of blood. One the days of heavy bleeding the contractions are so quick and strong that the blood gets moved out even before the anti-coagulants have time to start working. Hence, the reason for heavy dark colored clots of blood.
answered by C B