Unknowingly being intimate during Haidh

If a female has a fixed habit of 7 days Haidh (for example) and on termination of her Haidh (after the bleeding stops), she takes Ghusl and then engages in relations with her husband, then this will be in order. Thereafter, if she discovers blood after a day (on the 8th day) which is within the 10-day period of Haidh, it will be counted as Haidh, particularly if the change (from 7 days to 8 days) occurred after a 15-day clean period. Since, the husband and wife were not intentionally intimate during Haidh, but rather engaged in relations as they were convinced that the Haidh had terminated, there is hope that they will not be held accountable in the court of Allah. However, both parties should make Taubah as a precautionary measure.