I say, and with Allah's success: I'tikaf is not valid for a menstruating woman, a junub, or a postpartum woman; because purity from janabah, menstruation, and postpartum bleeding is a condition for the validity of I'tikaf. The junub, menstruating woman, and postpartum woman are prohibited from entering the mosque; as stated by the Prophet (peace be upon him): “I do not permit the mosque for a menstruating woman or a junub” in Sahih Ibn Khuzaymah 2: 284, Sunan al-Bayhaqi al-Kabir 2: 442, Sunan Abu Dawood 1: 60, and Musnad Ishaq ibn Rahawayh 3: 1032. This act of worship can only be performed in the mosque, so if a woman vowed to perform I'tikaf for a month and menstruated during it, she must leave the mosque and make up her days of menstruation and connect them to the month so that the continuity is not broken. If she does not connect them, she must start anew; because this amount of continuity is within her ability, while what has been dropped from her is known to be beyond her ability. However, if she vowed to perform I'tikaf for ten days and menstruated during it, she must start anew. Refer to: Al-Mabsut 3: 121, and Allah knows best.