Fourth: coagulation. Menstrual blood does not clot once it emerges, because it clotted in the uterus, then burst forth and flowed out; it therefore does not clot again. The blood of irregular bleeding, however, does clot, because it is venous blood. This is what some contemporary medical specialists have stated, and the Prophet ﷺ indicated this when he said: “It is blood from a vein.” It is well known that venous blood clots.
If it does not exceed the maximum duration of menstruation or fall short of its minimum, then it is her menstruation, which she observes in the second month, while the red blood is irregular bleeding. If her blood is not distinguishable, she observes the usual duration of menstruation each month.
His statement, “If it does not exceed its maximum,” means: if the black blood does not go beyond the maximum duration of menstruation, for if it exceeds that maximum, it cannot be considered menstruation.
Thus, if a woman has bleeding for twenty-five days—twenty days of it black and five red—the black blood cannot be considered menstruation, because it has exceeded the maximum duration of menstruation.
His statement, “or fall short of its minimum, then it is her menstruation, which she observes in the second month, while the red blood is irregular bleeding,” means: if the black blood does not fall short of the minimum duration of menstruation. Its minimum is one day and one night. Thus, if a woman menstruating for the first time says that on the first day the blood came to her it was black, then it became red for twenty days, she does not refer to discernment, because it cannot be considered menstruation, as it falls short of one day and one night.
But if she says that the black blood came for six days, then it is menstruation, because it neither falls short of its minimum nor exceeds its maximum; the remaining red blood is irregular bleeding.
His statement, “If her blood is not distinguishable, she observes the usual duration of menstruation,” means that the woman menstruating for the first time observes it.