Yevamot 35B

החולץ ליבמתו ונמצאת מעוברת וילדה, בזמן שהולד של קיימא ⁠— הוא מותר בקרובותיה, והיא מותרת בקרוביו, ולא פסלה מן הכהונה.

Mishnah: When a man who has a brother dies childless, his widow [yevama] and one of his brothers [yavam] may perform a ritual through which she is freed of her levirate bonds [ḥalitza]. It is then considered, with regard to forbidden relationships, as though they had been married and divorced. Therefore, he is forbidden to her relatives, and she to his. However, with regard to one who performs ḥalitza with his yevama and then she is found to have been pregnant at the time of the ḥalitza and she gave birth, in the event that the offspring is viable, the deceased husband has been survived by offspring and so there was never any levirate bond; consequently, the ḥalitza that was performed was entirely unnecessary and a meaningless act. As such, he remains permitted to her relatives and she remains permitted to his relatives. Furthermore, since the ḥalitza was meaningless, she is not afforded the status of a ḥalutza, i.e., a yevama who performed ḥalitza, a status akin to that of a divorcée. Therefore, the ḥalitza does not disqualify her from marrying into the priesthood.

אין הולד של קיימא ⁠— הוא אסור בקרובותיה, והיא אסורה בקרוביו, ופסלה מן הכהונה.

If the offspring is not viable, then it emerges that the ḥalitza was indeed necessary. Therefore, he is forbidden to have sex with her relatives and she is forbidden to have sex with his relatives, as though they had been married and divorced, and the ḥalitza disqualifies her from marrying into the priesthood, as she is afforded the status of a ḥalutza.

הכונס את יבמתו ונמצאת מעוברת וילדה, בזמן שהולד של קיימא ⁠— יוציא, וחייבין בקרבן.

With regard to one who consummates the levirate marriage with his yevama, i.e., he had sex with her under the assumption that there is a levirate bond and so there is a mitzva to do so, and then she is found to have been pregnant at the time of the sex and she gave birth, in the event that the offspring is viable the deceased brother has been survived by offspring and it is evident that there was never any levirate bond. In that case, the sex they had, rather than being a mitzva, were a violation of the prohibition against having sex with one’s brother’s wife. Therefore, the yavam must send her out, i.e., they must separate, as she is forbidden to him as his brother’s wife, and to atone for the forbidden sex that they had, they are each obligated to bring a sin-offering, as is the halakha for all who inadvertently transgress a prohibition that, when performed intentionally, is punishable by karet.