When discussing the moment of birth for purposes of determining when the ritual impurity of the mother begins, the Mishna (daf 28a) teaches:
If the embryo issued in pieces or in a reversed position it is deemed born as soon as its greater part issued forth. If it came forth in the normal way it is not deemed born until the greater part of its head issued forth. And what is meant by the issue of the ‘greater part of its head’? The issue of its forehead.
Thus, the Mishna distinguishes between cases where a child is born in a normal fashion, where the emergence of its head renders it “born” and cases where a child is born in an unusual fashion, and birth is determined based on the emergence of rubo – the majority of the body of the embryo.
What if we have an unusual birth, and the head emerges before the majority of the embryo’s body?
The Gemara on today’s daf discusses this question and brings a difference of opinion between Rabbi Elazar and Rabbi Yoḥanan. Rabbi Yoḥanan believes that the head determines birth, and whenever it appears – even in an unusual birth – the child is deemed to have been born. Rabbi Elazar rules that the law in the Mishna requiring that the majority of the embryo emerge in order for birth to take place applies even if the head emerges.
The Gemara points to a parallel discussion in a different area of halakha. How do we determine whether birth has taken place when determining the status of a firstborn?
According to the Gemara in Massekhet Bekhorot (daf 46b), Shmuel teaches that in the case of twins, if the first child’s head emerges briefly and then the second child is born, in the event that the first child was stillborn it would not be considered to be a birth, and the child that was born would retain its status as a firstborn. If, however, the child whose head briefly emerged was a living child, then the fact that its head came out first is significant, and it will be considered the firstborn.
While there are parallels between these cases, the Gemara concludes that the differences between a stillborn and an embryo that is born in pieces limits the connection between them.