What should a woman do if menstrual blood mixes with bleeding resulting from a surgical operation, making it impossible to distinguish between them?

If trustworthy doctors can determine whether the blood is menstrual blood or due to an illness, then their opinion should be followed. If they cannot determine this, the default is that the blood a woman sees is menstruation and not istihada (non-menstrual bleeding), provided it occurs at a time when menstruation is possible. The blood is considered menstruation as long as it does not exceed fifteen days; if it exceeds this, then it is istihada. Similarly, if the blood returns before the minimum period of purity (fifteen days according to the majority of scholars, and thirteen days according to the Hanbalis), and it cannot be considered part of the previous menstruation, then it is istihada. If it is established that the woman is experiencing istihada, she should refer to her previous habit. If she does not have a habit, then she should distinguish (between menstrual and non-menstrual blood). If she has neither a habit nor a distinguishing characteristic, she should consider the prevalent habit of women (six or seven days) as menstruation, and anything beyond that is istihada. She should perform ablution for each prayer after washing the area and taking protective measures.