What is the ruling on blood that flows for four or five days after marriage, breastfeeding, and insertion of an IUD, given that the customary period before that was seven or eight days, and it has started to flow at close intervals (every month, then every three weeks, then every two weeks)?

The minimum period of purity (Tuhr) between two menstrual cycles is thirteen days, according to the more preponderant opinion. So, if a woman sees blood for three or four days, then becomes pure for two weeks, and then the bleeding resumes, the returning blood is considered a new menstrual cycle.