Ohalos, Chapter 7, Mishnah 4. The Mishnah before us takes up a case that is at once halachically precise and deeply human, and through it teaches at what exact point a nefel (a child that did not emerge alive) begins to transmit tumah to the structure it is in. A woman is in the grip of a labor that will not progress, and in the middle of it she is transferred out of one dwelling and brought into a second one. The question is which of those two dwellings has become tamei.
The Case
The Mishnah opens: "Ha'isha shehi mikasha leiled", a woman whose labor is going badly and will not move forward. "Vayotzi'uha mibayis lebayis", for whatever reason she is taken out of where she was and brought into a second dwelling. It is there, in that second dwelling, that the child emerges, and it emerges without life.
Before we go further, a thought worth carrying with us. Reb Moshe writes in a teshuva that at times a neshama must come down into this world in order to receive its tikkun, and that tikkun can be accomplished even without the child ever drawing a breath in this world. The neshama came down, it received what it needed, and it returned. Nothing here is random or wasted, even when we cannot see the cheshbon.
The Halachah of the Two Houses
The ruling reads: "Harishon tamei besafek, vasheini bevaddai." As far as tumah and taharah are concerned, the dwelling she left carries a tumah of doubt, while the dwelling she entered is tamei with certainty.
Why is the first house only a safek? Because a lifeless child does not begin to convey tumah from the moment it is no longer alive. It conveys tumah only once it has descended and reached the stage that halachah calls being born, psichas hakever, the opening of the womb. Whether the child had already reached that stage while the mother was still in the first house we simply cannot know, so that house remains in doubt. In the second house there is no doubt at all: the child was actually born there, so the tumah is certain.
Here a natural question arises. If the child is no longer alive, why should it not convey tumah to the house immediately, even before it reaches that stage? The answer lies in where it is: it is inside its mother's body. This is what is called tumah belua, a tumah that is swallowed up inside something else. A tumah enclosed within a body does not spread outward, and so as long as the child has not reached the stage of being born, the house around the mother is unaffected.
Reb Yehuda's Clarification
The Mishnah continues: "Omar Reb Yehuda, eimatai?" Reb Yehuda asks when the first ruling applies. It is important to understand that Reb Yehuda is not disagreeing with the Tanna Kamma at all; he is coming to explain the Tanna Kamma's words. The Tanna Kamma said the first house is tamei out of doubt. Under what circumstances?
"Bizman shehi niteiles begapayeha", when she was carried out by her arms. That is, in the first house she had already reached a state in which she had no strength to walk on her own, and those with her had to take hold of her and support her as they brought her to the second house. In such a case there is real reason to suspect that the child had already begun to be born while she was still in the first house, and therefore that house is tamei besafek.
"Aval be'im haysa mehaleches, harishon tahor." But if she walked out of the first house on her own two feet, the first house is tahor, not even a safek, and everything in it remains clean. Why? "Shemishniftach hakever, ein penai lahalech." Once the womb has opened, once the child has reached the point that halachah calls being born, a woman no longer has the ability or the presence of mind to walk on her own. Therefore the very fact that she walked is a siman that the child had not yet reached that stage while she was in the first house.
The Size of the Head
The Mishnah then adds an important qualification, because there is a situation in which a mother could still walk even after the child reached that stage: if the child's head is unusually small. The Mishnah concludes: "Ein linefalim psichas hakever ad sheyagilu rosh kapika." A nefel does not open the womb in a way that prevents the mother from walking unless its head is like a pika, meaning as large as a pika (not literally as round as one, but of that size).
What is a pika? We met it in Maseches Keilim, in the discussion of the spindle used for spinning thread. The spindle is a long shaft with a sort of hook at the top, and lower down there is a small ball, a weight, through which the shaft passes and on which it sits, much the way one would push a marshmallow onto a stick. That little ball is the pika, and it is the measure our Mishnah is using.
So the logic runs like this. If the child's head is the size of a pika, then once it reaches the stage of being born the mother certainly cannot walk. In such a case, her having walked out on her own is a clear siman that the child had not yet begun to be born, and the first house is tahor. But if the head happens to be smaller than a pika, her walking proves nothing, since it is entirely possible that the child had already reached psichas hakever and she was still able to walk precisely because the head was so small. In that case the first house remains tamei besafek, even though she walked out on her own.
Reviewing the Mishnah
A woman labors with great difficulty and is transferred into a second dwelling, where the child emerges, its neshama having remained in shamayim rather than entering this world alive. The dwelling she left holds a status of doubt: a nefel transmits tumah only from the point at which it has descended to what halachah treats as birth, and we have no way of establishing whether that point had been reached before she was moved. The dwelling she was brought into is tamei with certainty, since that is plainly where the birth took place.
Reb Yehuda comes to clarify rather than to argue. The doubt attaches to the earlier dwelling in one circumstance only: she had to be lifted and carried out on the arms of those attending her, having no strength left to walk. In that situation it is entirely possible that the birth had already begun there. Should she have gone out under her own power, that itself serves as a siman that the process had not yet begun, and the dwelling she left is fully tahor. All of this, though, depends on the head of the nefel measuring a pika, that small weighted ball on the spindle. Where the head was smaller than that, walking would still have been possible for her even once the birth had begun, and so the doubt remains over the earlier dwelling notwithstanding that she left it on foot.