Mishnayos Yevamos Perek 4 Mishnah 2

With regard to one who consummates the levirate marriage with his yevama, i.e., he had intercourse 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 intercourse 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 relations they had, rather than being a mitzva, were a violation of the prohibition against engaging in relations 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 relations 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. And if the offspring is not viable, and therefore there was in fact a levirate bond, he may maintain her as his wife since his intercourse with her was a valid consummation of levirate marriage. If they consummated the levirate marriage and seven months later she gave birth, there is uncertainty whether the child is nine months old, i.e., counting from conception, and is the offspring of the first husband, and as such there was no levirate bond, or whether the child is only seven months old and is the offspring of the latter husband, i.e., the yavam, and not of the deceased, in which case there was a levirate bond. In that case, due to the possibility that she is forbidden to him as his brother’s wife, he must send her out. However, the lineage of the child is unflawed, since regardless of whether it was born of the first or second husband, there was no transgression involved in its conception. Furthermore, to atone for the possibility that they had forbidden relations they are both obligated to bring a guilt-offering for uncertainty, as is the halakha for anyone who is uncertain whether they inadvertently transgressed a prohibition that would require one to bring a sin-offering.

הַכּוֹנֵס אֶת יְבִמְתּוֹ, וְנִמְצֵאת מְעֻבֶּרֶת וְיָלָדָה, בִּזְמַן שֶׁהַוָּלָד שֶׁל קְיָמָא, יוֹצִיא וְחַיָּבִין בַּקָּרְבָּן. וְאִם אֵין הַוָּלָד שֶׁל קְיָמָא, יְקַיֵּם. סָפֵק בֶּן תִּשְׁעָה לָרִאשׁוֹן, סָפֵק בֶּן שִׁבְעָה לָאַחֲרוֹן, יוֹצִיא וְהַוָּלָד כָּשֵׁר, וְחַיָּבִין בְּאָשָׁם תָּלוּי: