What is the expiation for having sexual intercourse with one's wife during menstruation due to being unaware that the bleeding had ceased after a miscarriage?

The blood discharged due to a miscarriage before the embryo is formed (i.e., before 81 days) is istihaadah (irregular vaginal bleeding), not nifas (postpartum bleeding). Therefore, it does not prohibit prayer, fasting, or sexual intercourse, unless it coincides with the usual period of menstruation or exhibits its characteristics. It is not permissible for the husband to have intercourse with his wife before the bleeding stops and she performs ghusl. However, if this occurs unintentionally, there is no sin or expiation.