Two people commit zina, it results in pregnancy, they get married after the 4th month of the pregnancy before making tawbah to hide the sin, they live normal etc and then get divorced 3 times, the girl then tells the guy that their nikkah was invalid because they got married during her iddah (during the pregnancy) so based on this they want to marry, is it permissable for them?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

We have previously mentioned the difference of opinion among the scholars about the ruling on the marriage of a man with a woman with whom he has committed Zina before they both repent and before verifying that she is not pregnant. The preponderant opinion in our view is that this marriage is not permissible. But we mentioned that if such a marriage is concluded, it is effective taking into consideration the opinion of the scholars who hold that it is permissible. Please refer to Fataawa 83176 and 89292.

In any case, these three divorces are effective and that man is not allowed to take back his ex-wife until she marries another man in a serious marriage and then he divorces her (or dies).

With regard to the pregnancy which resulted from Zina, then if she gave birth to it after six months of marriage, then the child is attributed to her husband as he is the owner of the bed.

Finally, we say: (In Al-Fataawa Al-Kubra,) Ibn Taymiyyah warned us against following one’s desires in such cases as he said: “After the three divorces, no one should look at whether the bride's guardian was of good or bad moral character, so as to consider the bad moral character of the guardian as a sound reason for the divorce not being effective, as most jurists are of the view that the guardianship of a person of bad moral character is valid, and most of them are of the view that divorce takes place in such a marriage and even in other defective marriages.

“Assuming that the marriage is invalid, and that divorce does not take place concerning it, then it is permissible for someone who considers what is lawful as lawful to consider what is forbidden as forbidden, but none is allowed to believe what is lawful as forbidden. Such a husband used to consider that having sexual intercourse with his wife was lawful before divorce, and if she dies, he would inherit from her, and he considered that the marriage was valid, so how can he, after divorce, consider that his marriage was invalid?

“How could the marriage be valid if there is a benefit in its validity and invalid if there is a benefit in its invalidity. This view contradicts the consensus of all the Muslims, as they all agree that if someone believes that something is lawful, then he should believe this whether or not it is in conformity with his interest; and if someone believes that it is forbidden, then he should believe in this in both cases (i.e. before and after divorce). The spouses who get divorced do not think about the invalidity of the marriage due to the bad moral character of the guardian except in case of three divorces, and not in case of sexual enjoyment and inheriting from each other: at one time they follow the view of those who consider it as invalid and at another time they follow the view of those who are of the opinion that it is valid, according to their interest and desires. Indeed, this is not permissible according to the agreement of all the Muslims.” [End of quote]

Allaah Knows best.