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.
Scholars held different opinions regarding this issue:
The Shaafi‘i and Hanafi scholars believe that an adulteress has no ‘Iddah (i.e. waiting period to ascertain pregnancy); and accordingly it is permissible to carry out the marriage contract for a pregnant adulteress before she gives birth to her child; for adultery has no sanctity.
On the other hand, the Maaliki and Hanbali scholars believe that ‘Iddah is obligatory for the adulteress just as it is obligatory for any other woman (widow or divorcee). However, some scholars believe that she might wait for a month so as to ascertain whether she is pregnant or not. Accordingly, it is impermissible to marry an adulteress before spending her due ‘Iddah.
There is no harm if you adopt the first opinion, especially with the possibility of people being killed, as you have clarified, because of delaying carrying out the marriage contract. However, you should remind both parties; the adulteress and the adulterer, to repent to Allaah The Exalted; for according to the Hanbali school of Fiqh, the marriage of an adulteress and an adulterer is impermissible before their sincere repentance. Moreover, the majority of scholars highlighted the fact that repentance is not a condition for their marriage. Furthermore, you should observe the due conditions and requirements of the marriage contract to guarantee its validity, among which is the presence of the bride’s guardian and the witnesses.
However, there is still a problem in this regard; the illegitimate child of the adulteress and adulterer should not be attributed to the father i.e. the adulterer, instead he would be attributed to his mother only. This is the opinion of the majority of scholars. However, some scholars believe that if the adulterer married the pregnant adulteress, the child can be attributed to him.
Allaah Knows best.