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Suppose the mother's first offspring is delivered through the abdominal wall — יוצא דופן, essentially a Caesarean delivery — and the next offspring is delivered vaginally. Which one is the bekhor?
Rabbi Tarfon treats both as doubtful. The Caesarean offspring is first chronologically, but it is not פטר רחם, the one that "opens the womb." The second offspring really do open the womb, but it is not the mother's first offspring. Thus, each possesses only half of what we ordinarily mean by bekhor. The Gemara formulates Rabbi Tarfon's uncertainty beautifully:
בכור לדבר אחד אי הוי בכור
Is something that is a firstborn in one respect considered a firstborn?
Rabbi Akiva answers no. The first animal fails because אינו פטר רחם; the second fails because קדמואחר—another offspring preceded it. Thus, bekhorah requires the convergence of two biological/legal facts: first offspring + first emergence through the womb.
We explore What does "firstborn" mean when biological chronology and anatomical birth no longer coincide?
That is an unusually modern-sounding problem hiding in Bechoros 19. The Caesarean case forces חז"ל to distinguish chronological firstness, anatomical firstness, sexual firstness, and halakhic firstness. The body supplies several possible definitions of "first"; Torah law must determine which combination constitutes בכור.