Rule: If a woman bled for more than 10 days in the previous month, it is necessary to know the length of her most recent menstruation that did not exceed 10 days, that is, that did not involve istihada (non-menstrual bleeding). The length of her menstruation [for the month with blood exceeding 10 days] is calculated based on her most recent period that did not involve istihada.

Examples:

1. Supposing that a woman’s habit is 25 days of purity and 5 days of menstruation. If she, after remaining free from menstruation for 17 days, sees bleeding for 11 days in the 2nd month, it means that there is istihada as bleeding has exceeded 10 days. Because 3 days of bleeding [the 9th, 10th, 11th days] coincide with her previous menstrual period, these 3 days are considered menstrual bleeding, and the first 8 days are considered istihada. Her purity period does not change, but her menstrual period has decreased to 3 days. She must make up the salats that she missed during the days of istihada later on. When she calculates the duration of her menstruation for the following months, she will not take the duration of purity and bleeding in the 2nd month into consideration as it involves istihada. The month with valid menstruation [the 1st month when she experienced 25 days of purity and 5 days of menstruation] will be taken as the standard.

2. If the same woman remains free from menstruation for 18 days and then sees bleeding for 11 days in the 3rd month, it means that there is istihada as bleeding has exceeded 10 days. Because 4 days of bleeding [the 8th, 9th, 10th, 11th days] coincide with the days of menstruation of the first month [25 days of purity and 5 days of menstruation], which did not involve istihada, these 4 days are considered menstruation, and the first 7 days are considered istihada.

3. If the same woman remains free from menstruation for 27 days and then sees bleeding for 12 days in the 4th month, it means that there is istihada as bleeding has exceeded 10 days. Because 3 days of bleeding [the 1st, 2nd, and 3rd days] coincide with the days of menstruation of the first month [25 days of purity and 5 days of menstruation], these 3 days are considered menstrual, and the remaining 9 days are considered istihada.

4. If the same woman remains free from menstruation for 30 days and then sees bleeding for 11 days in the 5th month, it means that there is istihada again as bleeding has exceeded 10 days. Because none of the days of bleeding in this month coincides with the days of menstruation of the first month [25 days of purity and 5 days of menstruation], the length of the first month’s menstruation is taken as the standard. The duration of her menstruation in the first month was 5 days. The duration of menstruation for this month is considered 5 days, too. The remaining 6 days are counted as istihada.

5. If the same woman remains free from menstruation for 28 days and sees bleeding for 11 days in the 6th month, it means that there is istihada as bleeding has exceeded 10 days. Because 2 days of bleeding coincide with the days of menstruation of the first month, these 2 days are treated as unimportant because 3 days at least must coincide with her former menstruation in order that they may be thought as menstruation. The length of her menstruation in the first month is taken as the standard. It was 5 days, and her menstruation for this month is 5 days. The remaining 6 days are counted as istihada.

6. If the same woman remains free from menstruation for 27 days and then sees bleeding for 10 days in the 7th month, it means that there is no istihada as bleeding has not exceeded 10 days. In this case, none of her former menstrual periods is taken into consideration. Her norm of menstruation has changed to 10 days. Her purity period has increased to 27 days.

7. If the same woman remains free from menstruation for 18 days and sees bleeding for 11 days in the 8th month, it means that there is istihada as bleeding has exceeded 10 days. Because 2 days of bleeding coincide with her menstruation in the 7th month, which is her most recent, valid menstruation that does not involve istihada, they are treated as unimportant. In this case, the length of her menstruation in the 7th month is taken as the standard. It was 10 days, and her menstruation for this month is counted as 10 days. Day 11 is considered istihada.