Is sexual intercourse forbidden on days when menstruation is expected, knowing that the bleeding has not yet occurred, and that its onset follows intercourse directly or is slightly delayed?

A woman is not considered to be menstruating unless she sees the known menstrual blood, which appears when she sits down to relieve herself. So, she should not abstain from marital intimacy with her husband or abandon prayer on the pretext of her period being near, as long as she has not seen it. If intercourse occurs during menstruation out of ignorance, there is no sin.