How is the 'iddah calculated for a divorced woman if *quru'* means periods of purity (*tuhur*)?
General Chapter
Al-Mughni
Book of Waiting Periods ('Iddah)
Primary text
If the divorce occurs while she is pure, her 'iddah concludes upon seeing the blood of the third menstruation. If the divorce occurs while she is menstruating, her 'iddah concludes upon seeing the blood of the fourth menstruation. This is the opinion of Zayd ibn Thabit, Ibn 'Umar, 'A'ishah, Al-Qasim ibn Muhammad, Salim ibn 'Abdullah, Aban ibn 'Uthman, Malik, and Abu Thawr, and is the apparent position of the Shafi'i school. The evidence rests on the statements of these Companions, and the Hadith of Zayd ibn Thabit stating that once she enters the blood of the third menstruation, she is free from him, and they do not inherit from each other.
Supporting text
A secondary opinion held by Shafi'i and within our school suggests that the 'iddah does not conclude until one day and one night have passed after seeing the blood, to guard against the possibility that the discharge is *dam fasad* (irregular bleeding, not true menstruation), thus requiring certainty before ruling the 'iddah complete. This is countered by the fact that the discharge has already been ruled a valid menstruation concerning the cessation of prayer and prohibition of the husband, so it must also be counted for the 'iddah. If certainty is required, once it is confirmed to be menstruation, the 'iddah is deemed completed from the moment she first saw the blood.