Ketubot, Chapter One, Mishnah Nine
In this mishnah a single woman is discovered pregnant, and it is unknown to others who the father is. If the father was from those prohibited from marrying Israelites, then the child will follow his status. Furthermore, the woman will also be prohibited from subsequently marrying a priest (as in yesterdays mishnah). Again, the rabbis debate whether or not the woman is believed.
She was pregnant and they said to her, What is the nature of this fetus?
[And she answered, It is] from so-and-so and he is a priest.
Rabban Gamaliel and Rabbi Eliezer say: she is believed,
And Rabbi Joshua says: we do not live by her mouth, rather she is in the presumption of having had relations with a natin or a mamzer, until she brings proof for her statement.
This mishnah is nearly identical to yesterdays mishnah, the only difference being that this woman is pregnant. The reason why the mishnah reiterates the positions outlined in the previous mishnah is to teach that Rabban Gamaliel and Rabbi Eliezer believe the woman even if she is pregnant. In yesterdays mishnah, it was unclear whether or not she had even had sex with the man in question. When asked who he was, she could have said that she never had relations with him. Therefore, when she admitted that she did but said that she was a priest (i.e. one who is allowed to marry an Israelite), she is believed. However, in todays mishnah it is certain that she had relations with someone and she could not make a better claim than to say that the man was fit to marry an Israelite. [This type of reasoning is common in the mishnah, and it is called migo, which means that when a person could have made a better claim, he is believed when he makes a worse claim]. Nevertheless, Rabban Gamaliel and Rabbi Eliezer hold that she is believed.
A further innovation is that not only is the woman believed, and she is subsequently allowed to marry a priest, but her child is assumed to be fit to marry an Israelite. In other words, even though we dont know for sure that the child is not a mamzer or a natin, the law treats him/her as if he was not.
Rabbi Joshua again states that the woman is not believed. Furthermore, her child is assumed to be the child of a natin or a mamzer and may not marry an Israelite until s/he proves otherwise.