Istihaadah does not prevent i'tikaf (spiritual retreat). If the woman experiencing istihaadah is confident that she will not cause impurity, she does not leave her i'tikaf. If she does leave, her i'tikaf is invalidated. Aisha (may Allah be pleased with her) said: "A woman from the wives of the Messenger of Allah (peace be upon him) was in a state of istihaadah while I was in i'tikaf with him. She would see the redness and yellowness, so we would sometimes place a basin under her while she prayed." This is reported in Sahih al-Bukhari 2: 716, and in Sunan al-Bayhaqi al-Kabir 1: 328. Refer to Bada'i al-Sana'i 2: 108, and the rulings and importance of i'tikaf, p. 57.