What is the standard for determining menstruation when the duration is unknown or when blood appears outside the customary time?

Primary text

The standard for determining menstruation is the actual flow of blood itself, relying on the common knowledge ('urf) among women, rather than strictly adhering to a prior established calendar habit, provided the duration does not exceed the maximum length of menstruation. This is evidenced by the Prophet’s instruction to a wife who experienced bleeding to wrap a cloth and determine if she was menstruating, without inquiring about her established schedule. Similarly, when Aisha experienced her menses during Umrah, the determination was based solely on seeing the blood, not on a pre-existing habit. Adhering rigidly to a repeated habit for shifts would lead to situations where women might be deprived entirely of being deemed menstruating despite seeing blood that qualifies as menstruation.