Responding to this issue Dr. Hatem Al-Haj, Professor of Fiqh he said that ‘ a woman who commited zina during her waiting period should repent to Allah a sincere repentance,because she committed adultery, which is a horrendousenormity.
In this case, her waiting period from the first man (thehusband), will not end until she gives birth. Afterwards, ifshe and the fornicator repented, they may marry oneanother.
If the husband is certain the child is not his, he shoulddecline its paternity. If he didn’t, then it is his since it wasconceived while she was still a wife. (A woman in theperiod takes the rulings of the wife in most regards).