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Our Mishnah claims that if a father said, “This son of mine is a mamzer,” he is not believed (since the father is related to his son, he is therefore disqualified from testifying about him). And even if both of them (the father and the mother) admit regarding the fetus in her womb (that she became pregnant from some other man), they are not believed. Rabbi Yehudah said: They are believed.
Believing the father
The Gemora explains the Mishna: The father is not believed for he is not certain from whom she became pregnant. And even the mother (who is certain) is also not believed. And even in a case where the son does not have any presumption of legitimacy (in a case where he has not yet entered this world), they are still not believed.
We explore the testimony of parents vs children and the nature of eidus.