יורה דעה שי״חYoreh De'ah 318

Se’if 1א׳

רָחֵל שֶׁלֹּא בִּכְּרָה וְיָלְדָה שְׁנֵי זְכָרִים, אֲפִלּוּ יָצְאוּ שְׁנֵי רָאשֵׁיהֶם כְּאֶחָד, אִי אֶפְשָׁר שֶׁלֹּא קָדַם אֶחָד, הוֹאִיל וְאֵין יָדוּעַ אֵיזֶה יָצָא רִאשׁוֹן, הַכֹּהֵן נוֹטֵל אֶת הַכָּחוּשׁ וְהַשֵּׁנִי סְפֵק בְּכוֹר. מֵת אֶחָד מֵהֶם, אֵין לַכֹּהֵן כְּלוּם, שֶׁזֶּה הַחַי סָפֵק הוּא וְהַמּוֹצִיא מֵחֲבֵרוֹ עָלָיו הָרְאָיָה. וְכֵן אִם יָלְדָה זָכָר וּנְקֵבָה, הֲרֵי הַזָּכָר סָפֵק שֶׁמָּא הַנְּקֵבָה יָצְאָה תְּחִלָּה, לְפִיכָךְ אֵין לַכֹּהֵן כְּלוּם שֶׁהַמּוֹצִיא מֵחֲבֵרוֹ עָלָיו הָרְאָיָה.

A ewe that has not yet borne, and she bore two males — even if both their heads came out as one, it is impossible that one did not come first; since it is not known which came out first, the priest takes the lean one, and the second is a doubtful firstborn. If one of them died, the priest has nothing, for this living one is a doubt, and the burden of proof lies on the one who seeks to take from his fellow. And likewise if she bore a male and a female, the male is a doubt, lest the female came out first; therefore the priest has nothing, for the burden of proof lies on the one who seeks to take from his fellow.

Se’if 2ב׳

שְׁתֵּי רְחֵלוֹת שֶׁלֹּא בִּכְּרוּ וְיָלְדוּ שְׁנֵי זְכָרִים, שְׁנֵיהֶם לַכֹּהֵן, זָכָר וּנְקֵבָה, הַזָּכָר לַכֹּהֵן; שְׁנֵי זְכָרִים וּנְקֵבָה, הַכֹּהֵן נוֹטֵל אֶת הַכָּחוּשׁ וְהַשֵּׁנִי סְפֵק בְּכוֹר; וְאִם מֵת אֶחָד מֵהֶם, אֵין לַכֹּהֵן כְּלוּם, שֶׁזֶּה הַחַי סְפֵק בְּכוֹר הוּא וְהַמּוֹצִיא מֵחֲבֵרוֹ עָלָיו הָרְאָיָה.

Two ewes that have not yet borne, and they bore two males — both are the priest's. A male and a female — the male is the priest's. Two males and a female — the priest takes the lean one, and the second is a doubtful firstborn; and if one of them died, the priest has nothing, for this living one is a doubtful firstborn, and the burden of proof lies on the one who seeks to take from his fellow.

Se’if 3ג׳

יָלְדוּ שְׁתֵּי נְקֵבוֹת וְזָכָר, אוֹ שְׁנֵי זְכָרִים וּשְׁתֵּי נְקֵבוֹת, הֲרֵי הַזְּכָרִים סָפֵק, שֶׁאֲנִי אוֹמֵר: שֶׁמָּא הַנְּקֵבָה נוֹלְדָה תְּחִלָּה וְאַחַר כָּךְ הַזָּכָר, לְפִיכָךְ אֵין כָּאן לַכֹּהֵן כְּלוּם, שֶׁהַמּוֹצִיא מֵחֲבֵרוֹ עָלָיו הָרְאָיָה.

If they bore two females and a male, or two males and two females, the males are a doubt, for I say: perhaps the female was born first and afterwards the male; therefore the priest has nothing here, for the burden of proof lies on the one who seeks to take from his fellow.