Arachin, Chapter 1, Mishnah 4. The mishnah before this one presented a person whom the Beit Din has already condemned in a capital case, and taught that such a person stands outside the laws of arachin entirely. Our mishnah remains with that very figure and adds several further rulings that concern him, and her.
A pregnant woman who is led out to be executed
The mishnah begins with a pregnant woman: "Ha'ishah sheyotze'ah lehaharag", a woman being led out to her execution, "ein mamtinin lah", we do not hold the sentence back, "ad sheteled", until she has delivered. The fact that she carries a child in her womb does not defer anything: the verdict is carried out at once, and the unborn child dies with her.
Why is this not a separate death being caused? Because mother and fetus are treated here as a single entity, one body. The fetus is not regarded as an independent life; it is considered part of the mother. This is the principle of ubar yerech imo, that a fetus is like the thigh of its mother, a limb of her body. Certainly lechatchilah, when there is a choice in the matter, one may not destroy a fetus. But in this situation, where the mother herself is condemned, the child in her womb shares her fate and is put to death along with her.
Once she has sat on the birthing stool
The mishnah goes on: "Yashvah al hamashber", if she has already taken her place upon the birthing stool, that is, labor has set in, then "mamtinin lah", the sentence is held back, "ad sheteled", until the child is delivered. Once labor has begun the child has moved from where it lay; it is no longer settled inside the womb but already making its way into the world. From that point it counts as a being unto itself rather than a limb of its mother, and so the execution is deferred until the delivery, lest we destroy a life that has become separate from hers.
Benefit from the hair of a woman who was executed
The mishnah now moves to a different question. It is forbidden to derive any benefit from a corpse, and this issur holds no less for a person whom the court has executed. The case before us concerns added hair. A woman whose own hair grew thin would braid extra hair into it, a kind of wig or partial hairpiece, so that her head would look fuller and more becoming. If she was executed while that hairpiece was still woven into her hair, may anyone make use of it?
The ruling is: "Ha'ishah shenehergah, nehenin bise'arah". Where a woman has been executed by sentence of the court, the hair she was wearing when she died may be used and enjoyed.
The Chachamim in the Gemara raise a difficulty. If the added hair was tied and interwoven with her own hair, it should have the status of her own hair, since it has become part of it, and benefit from it should be forbidden. The Gemara therefore explains that the mishnah must be speaking of a case where the woman had made her intention known before the execution, stating that she wished this hairpiece to go to her daughter or to someone else. That is galeh da'atei, she revealed her mind on the matter, showing that she did not want this additional hair to remain part of her body. Consequently we view it as though it had been detached from her at the moment of death, and benefit from it is permitted.
Where an animal has been condemned
With a beast the ruling runs in the opposite direction. The mishnah teaches: "Behemah shenehergah, asurah behana'ah". An ox or other animal that took a human life and received a capital verdict from the court is forbidden in benefit. No part of it may be enjoyed, and the issur takes hold already when the verdict is pronounced, before the sentence has been carried out. Even its hair, which one might dismiss as a trifling detail of the animal, is closed off to any use whatsoever.