Bekhorot 19B

אי למעוטי היכא דיצתה נקבה לפניו ⁠— מ״פטר רחם״ נפקא! אלא שמע מינה ״בכור״ למעוטי היכא דיוצא אחר יוצא דופן.

If it serves to exclude a case where a female emerged from the womb before it, that is derived from the phrase “that which opens any womb.” Rather, conclude from it that “firstborn” serves to exclude a case where an animal emerged from the womb after its older sibling was born by caesarean section. According to Ravina, the word “firstborn” is referring to an animal that is a firstborn even in only one aspect, but as it is extraneous it serves to exclude an animal whose older sibling was born by caesarean section.

אמר ליה רב אחא מדפתי לרבינא: אי סלקא דעתך בכור לדבר אחד הוי בכור, תינח היכא דיצא זכר יוצא דופן וזכר דרך רחם, דלא קדוש, דאימעוט לידה מ״בכור״, דבכור לרחמים איכא, בכור לזכרים ליכא.

Rav Aḥa of Difti said to Ravina: If it enters your mind that a firstborn in one aspect is called a firstborn and the derivation is based on the fact that the term “firstborn” is superfluous, that works out well in a case where a male emerged by caesarean section and then another male emerged through the womb. It stands to reason that it is not sanctified, as this birth is excluded by the word “firstborn,” as it is the firstborn of the womb, but it is not the firstborn of the males.

אלא היכא דיצתה נקבה דרך דופן וזכר דרך רחם ⁠— ליקדש, דהא איכא בכור לזכרים ובכור לרחם! אלא מחוורתא כדאביי.

But in a case where a female emerged by caesarean section and afterward a male was born through the womb, let it be sanctified, as it is the firstborn of the males and the firstborn of the womb. One cannot derive the exclusions of both of these cases from the word “firstborn,” and yet the baraita indicates that even if a female was born first by caesarean section, the male born afterward is not considered the firstborn. The Talmud concludes: Rather, it is clear that the baraita must be interpreted in accordance with the explanation of Abaye, that a firstborn in one aspect is not called a firstborn.