Praise be to Allaah.
This marriage is considered to be null and void, because it happened when she was pregnant as a result of zinaa, and because it was lacking in one of the essential conditions of marriage, which is the presence of two male witnesses and the acceptance of her guardian. On this basis, the marriage contract must be repeated with a Muslim walee (guardian) or qaadi (judge). The children should be named after the father on whose bed they were born and the father should not disown any one of them, because “the child is for the bed.”
And Allaah knows best.