Related for Ketubot 26:5

If she were pregnant and they said to her: "What is the nature of this foetus?", [and she said:] "It is from Ploni the kohen"—Rabban Gamliel and Rabbi Eliezer say: She is believed since this is testimony that a woman is fit for. Rabbi Yehoshua says: She is not believed. Rabbi Yehoshua said to them: Do you not agree that, regarding a captive woman who was captured among the Gentiles, who has witnesses that she were captured, and she says "I am pure" that she is not believed!? They said to him: No! You would say this with a captive woman who has witnesses [that she was captured], can you say it about this one who has no witnesses [that she had a child with this man]!? He said to them: But what greater testimony is there than this, that her womb is between her teeth!? He said to them: There is no guardian for illicit sexual relations [i.e. women aren't expected to check the lineage of their sexual partners, so we have no idea if the father of the foetus is in fact pasul]. When does this apply [that a woman's testimony can be believed]? For testimony about her body [i.e. whether she can go onto marry a kohen or not], but with the child everyone agrees that he is a shetuki [a genealogical blemish belonging to someone whose father is unknown].