Mishnayos Yevamos Perek 4 Mishnah 1

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. If the offspring is not viable, then it emerges that the ḥalitza was indeed necessary. Therefore, he is forbidden to engage in relations with her relatives and she is forbidden to engage in relations 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.

הַחוֹלֵץ לִיבִמְתּוֹ, וְנִמְצֵאת מְעֻבֶּרֶת וְיָלָדָה, בִּזְמַן שֶׁהַוָּלָד שֶׁל קְיָמָא, הוּא מֻתָּר בִּקְרוֹבוֹתֶיהָ, וְהִיא מֻתֶּרֶת בִּקְרוֹבָיו, וְלֹא פְסָלָהּ מִן הַכְּהֻנָּה. אֵין הַוָּלָד שֶׁל קְיָמָא, הוּא אָסוּר בִּקְרוֹבוֹתֶיהָ, וְהִיא אֲסוּרָה בִקְרוֹבָיו, וּפְסָלָהּ מִן הַכְּהֻנָּה: