Bechoros Daf 46 בכורות דַף 46

The eighth Perek begins: יש בכור לנחלה ואינו בכור לכהן – there is a firstborn which is considered a bechor for inheritance but not a bechor for the Kohen (i.e., he receives a double portion of his father’s estate, but does not require פדיון הבן); there is a bechor for the Kohen but not for נחלה; there is a bechor for both; and there is a child who is not a bechor for either. This Mishnah defines the first two categories.

הבא אחר נפלים שיצא ראשו חי – If one [is born] after a נפל (premature baby) whose head emerged while it was alive, and then retracted, the fully developed twin born afterwards is not a bechor for the Kohen, since the נפל was considered “born” and the first to “open the womb.” Still, the viable twin is a bechor for נחלה, which is determined by "ראשית אונו" – [the father’s] initial strength, which is darshened as מי שלב אביו דוה עליו – one over whom the father’s heart would grieve, i.e., a live and viable child. Since the first child was a נפל, the second child is the first for whom the father would grieve. The same applies if the first child was fully developed but died before its head emerged.

Shmuel said: אין הראש פוטר בנפלים – the emergence of the head of a נפל does not exempt its fully developed twin from פדיון הבן, i.e., it is not considered “opening the womb.” This is because the passuk says כל אשר נשמת רוח חיים באפיו – all in whose nostrils was the breath of the spirit of life, implying: כל היכא דנשמת רוח חיים באפיו – wherever there is the breath of the spirit of life, i.e., it is viable, its birth is determined by its nostrils (i.e., its head); הוא דחשיב רישיה – then its head is significant to be considered birth, but the head of a nonviable child is not significant that its emergence should be considered birth.

Shmuel’s ruling is refuted from our Mishnah, which states that a child born after a nefel שיצא ראשו חי – whose head emerged while it was alive is not a bechor for the Kohen, because the נפל was considered “born”!? Although the Gemara suggests the Mishnah means that "רובו" – [the baby’s] majority emerged, this is ultimately rejected.

A Mishnah teaches that a child born headfirst is considered born once "רוב ראשו" – most of its head emerges, which it defines as משתצא פדחתו – from when its forehead emerges. Reish Lakish says: פדחת פוטרת בכל מקום חוץ מן הנחלה – the emergence of the forehead constitutes birth and exempts its twin from bechor status in all cases except regarding inheritance (i.e., the next born child will receive a double portion). This exception is because the passuk there says "יכיר" – he must recognize, and Reish Lakish holds one can only be recognized through his facial features. Rebbe Yochanan says the forehead is even considered birth regarding נחלה, because he holds one can be recognized by his forehead alone.

Reish Lakish’s expression that the פדחת exempts "בכל מקום" means to include a Baraisa’s ruling, that if the forehead of a גיורת’s child emerged before she converted, she does not observe the periods of tumah and taharah which follow a Jewish birth, nor does she bring the korban brought by a Jewish woman after giving birth.