Is expiation (kaffarah) obligatory for a husband if he has intercourse with his wife on the ninth day after the cessation of bleeding, and then the bleeding returns on the tenth day, knowing that the duration of menstruation increased due to a contraceptive method? Is this increase considered menstruation or istihadah (non-menstrual vaginal bleeding)?

The maximum duration of menstruation, according to the majority of scholars, is fifteen days. So, as long as the total blood does not exceed this period, all of it is considered menstruation, even if it exceeds the woman's usual duration. There is another opinion which states that what exceeds the usual duration is not considered menstruation until it recurs three times. The first opinion is the more preponderant one. If purity intervenes between the days of bleeding, then it is a valid state of purity, during which the woman performs ghusl (definition), prays, and intercourse is permissible. If the blood returns, she reverts to being in a state of menstruation, as long as it is within the time frame possible for menstruation.