2,183. "You Are Not the Father"

Hilchos Issurei Biah 15:18

Let’s say that people are gossiping about a betrothed woman, saying that she was promiscuous with her fiancé and with others. In such a case, her child is a doubtful mamzer. This is so even if her fiancé was intimate with her in her father’s house. Just as she acted promiscuously with her fiancé, she may have acted promiscuously with others. If she was questioned and reported that the baby was fathered by hef fiancé, the child is valid as has already been explained.

Hilchos Issurei Biah 15:19

If a married woman is pregnant and says that the baby isn’t her husband’s, her word isn’t accepted to invalidate the child; rather, we presume that the child is valid because the Torah only mandate’s the father’s word on this subject. If the father says that the baby isn’t his or if he’s abroad, then the child is presumed to be a mamzer. If the woman claims to have been impregnated by a non-Jew or a servant, the child is valid because the husband is incapable of contradicting her. A fetus won’t stay in its mother’s womb for more than 12 months (this is a justification to not deem a child a mamzer).