2,182. The Child of a Betrothed Woman

Hilchos Issurei Biah 15:16

Just as a man’s word is accepted to identify a particular son as his firstborn, his word is also accepted to identify his son as a mamzer, the son of a divorcee or the son of a woman who had performed chalitzah (the ceremony that severs a levirate bond). Similarly, if his wife is expecting, his word is accepted if he says that the baby isn’t his and is a mamzer. In such a case, the child is deemed a definite mamzer. If a man identifies himself as a mamzer, his word is accepted to the prohibit him from marrying a woman who was born Jewish but not to permit him to marry a mamzeres until he is identified as a definite mamzer. The same is true of his son. His word is not accepted to invalidate his grandchildren. A person can only invalidate himself.

Hilchos Issurei Biah 15:17

Let’s say that a betrothed woman becomes pregnant while still living in her father’s house. Such a child is presumed to be a mamzer and may marry neither a woman who was born Jewish nor a mamzeres. If his mother claims to have been impregnated by the man who betrothed her, she is taken at her word and the child is deemed valid. If her fiancé contradicts her and says that they were never intimate, then the child is deemed a mamzer. Even if the child was presumed to be his, a man’s word is taken if he says that his son is a mamzer. Nevertheless, the woman is not deemed a zonah (one who has engaged in intimacy with a prohibited relation). Rather, her word is taken if she says that she was intimate with her fiancé. Since she’s not ruled a zonah, she need not divorce her husband if she married a kohein and any children she had with him are valid kohanim.