Bekhorot 46B

גמ׳ אמר שמואל: אין הראש פוטר בנפלים.

Talmud: Shmuel says: In a case where the head of a fetus emerged and then went back into the womb, the offspring is not considered to have been born and does not exempt the next fetus from the obligation of redemption of the firstborn, e.g., if its twin brother was born first. Shmuel says this halakha specifically in a case of miscarriage, i.e., where the fetus whose head emerged was a stillborn and the one that was eventually born first is a viable offspring. But in a case where both are viable offspring, the emergence of the head is considered birth.

מאי טעמא? ״כל אשר נשמת רוח חיים באפיו״⁠ — כל היכא דנשמת רוח חיים באפיו הוא דחשיב רישיה, אידך לא חשיב רישיה.

The Talmud asks: What is the reason for Shmuel’s ruling? The verse states: “All in whose nostrils was the breath of the spirit of life” (Genesis 7:22), from which it is derived: Anywhere that one has the breath of the spirit of life, i.e., if it is viable, one goes by its nostrils, i.e., its head is considered significant. But with regard to another offspring, one that is not viable, its head is not considered significant.

תנן: הבא אחר נפלים, שיצא ראשו חי, ובן תשעה שיצא ראשו מת. קתני מיהא ״ראשו!״ מאי ״ראשו?״ רובו.

The Talmud raises a difficulty: We learned in the Mishnah: What is a firstborn with regard to inheritance but not with regard to redemption from a priest? It is a son who came after the miscarriage of an underdeveloped fetus, even where the head of the underdeveloped fetus emerged alive; and a fully developed 9-month-old fetus whose head emerged dead. Although the Mishnah addresses a 9-month-old fetus as well, in any event it teaches with regard to a miscarriage that its head exempts the offspring born after it. The Talmud responds: What is the meaning of the term: Its head? It means its head and most of the body.