What is the ruling on sexual intercourse if a woman discovers after it that she was menstruating?

If intercourse occurs before a woman becomes aware of her menstruation, there is no sin or expiation, due to Allah's saying: "Our Lord, do not impose blame upon us if we have forgotten or erred," and His saying: "And there is no blame upon you for that in which you have erred, but [blame is due for] what your hearts have intended." This is the view of the majority of scholars.

Thus, there is nothing upon the spouses as long as menstruation did not become apparent until after intercourse. If the blood flow exceeds fifteen days, it is considered istihadah (non-menstrual vaginal bleeding), and intercourse with a woman experiencing istihadah is permissible.