the scholars say: It is a vein that bursts forth from the lower part of the womb, whereas menstruation comes from the depth of the womb; and it is a stroke from Satan, by which he seeks to spoil a woman’s worship and cast her into doubt and confusion. This is a matter known to us only through revelation.

This irregular bleeding that a woman experiences falls into one of three cases:

First: she has an established menstrual habit. She observes the days of her customary period, then performs ghusl and prays, doing as women in a state of purity do.

Second: she has no established habit, or she has forgotten her habit, but she is able to distinguish the blood. In that case, she relies on this distinction. The scholars have mentioned three signs of menstrual blood, and some physicians have mentioned a fourth. The three signs are blackness, thickness, and foulness—that is, an unpleasant odor. Menstrual blood is black, foul-smelling, and thick, whereas the blood of irregular bleeding is red, thin, and odorless. This is a clear distinction.

Some contemporary physicians have mentioned a fourth difference: menstrual blood does not clot, whereas the blood of irregular bleeding does clot. They explained this by saying that menstrual blood had already clotted in the womb before being discharged, and therefore does not clot again; unlike the blood of irregular bleeding, which issues from a vein and is like other blood. Blood that comes from veins clots.

Thus, if her menstrual habit is inconsistent, or she has forgotten it, for example, or irregular bleeding began with her from the outset, she follows the distinguishing signs.