How is menstruation determined for a woman with a regular established habit (Mu'tadah)?
Chapter on Menstruation
Al-Mughni
Book of Purification
Primary text
If a woman has a regular habit, such as five days of menstruation at the beginning of every month, she must cease prayer (and fasting, implicitly) from the first day she sees blood in that usual time and must perform ghusl (ritual bathing) when the flow ceases. Subsequent rulings depend on whether the intervening purity inside the flow is considered to prevent subsequent flow from being counted as menses. If purity prevents resumption, her menses is only the first day, and subsequent flow is Istihadah. If purity does not prevent resumption, her menses includes the first, third, and fifth days, totaling three days of her usual cycle, with the rest being Istihadah. A third view suggests all five days of flow within her habit are counted as menses, making her sit for the seventh and ninth days if those are days of expected flow.
Supporting text
The sounder opinion dictates that she does not combine (talfeeq) days outside her usual habit, meaning if purity breaks the flow, subsequent blood is not added to the initial days if those days are outside her established pattern. If a woman is distinguishing (Mumayyizah), she sits for the duration of the dark blood, which constitutes her menses, and the remainder is Istihadah.