How is menstruation calculated for a woman whose bleeding exceeds the maximum period of menstruation, who cannot distinguish the blood's characteristics and has forgotten the number of days of her previous cycle, but remembers when it began?

Summary

This woman falls under the seventh category of mustahadah, a woman with a habit who cannot distinguish blood and remembers the timing of her habit but not its length. Days that are certainly menstruation are treated as menstruation, and days that are certainly pure are treated as purity, as for an istihadah woman. Days that may be menstruation or purity require caution (ihtiyat), and ablution for each obligatory prayer is sufficient. Days that may be menstruation, purity or a break in bleeding require a full bath before each obligatory prayer.

Written in our own words from the original fatwa — check the source before acting on it.