What are the days that are considered menstruation and those that are considered istihada (non-menstrual bleeding) from a 13-day bleeding period due to an IUD, given the difference in the nature of the blood and discharge during it, and are prayer and intercourse permissible during this period?

Brown spots and discharges before the onset of regular, heavy menstrual bleeding are not considered menses. Similarly, brownish or yellowish discharge (kudrah or sufrah) after the blood flow is not considered menses unless it is continuous with it. Therefore, if a woman first sees brownish discharge, she should not consider it menses until she sees regular blood. When she sees the blood, she should stop praying and fasting and remain in that state until the blood, and any attached yellowish or brownish discharge, ceases. When it stops and she observes dryness (al-jufuf), she should perform ghusl and pray. If yellowish or brownish discharge recurs after that, she should not consider it menses, and intercourse is permissible.