Niddah, Chapter 3, Mishnah 5. This chapter takes up the question of which forms of miscarriage carry with them the tumah of childbirth, and how a woman counts her days of tumah and her days of taharah when the identity of what emerged is unclear. Our mishnah presents two such cases: a fetus whose shape is unrecognizable, and a placenta that emerged with no visible child at all.
The mishnah opens: "Hamapeles sandal o shilya." A woman miscarries something with the flattened form of a shoe, which is what the mishnah calls a sandal, or she discharges a shilya, a placenta.
The ruling is "teshev lezachar velenekeivah." We assume that a child was indeed born here, but since there is no way to determine whether it was a boy or a girl, she must accept upon herself the stringencies that apply to each. In practice this means that she keeps fourteen days of tumah, the longer period that follows the birth of a girl, and then she has only twenty six further days of taharah, so that her entire period comes to a close at the fortieth day, which is the shorter span that follows the birth of a boy. The leniencies of each possibility cancel each other out, and she is left with the demands of both.
A second case follows: "Shilya babayis, habayis tamei." When a placenta is found lying inside a house, the whole house takes on the status of tamei. What lies behind this? The rule stated as "ein shilya belo vlad": a placenta never develops on its own, and a child must have been enclosed within it. The house is therefore judged as one that holds a dead infant, Rachmana litzlan, with every consequence that such tumah brings.
The mishnah is careful to spell out the reasoning: "Lo shehashilya vlad, ela she'ein shilya belo vlad." The placenta itself is not a child and carries no tumah of its own. The tumah of the house stems entirely from the certainty that a child accompanied it.
Rebbi Shimon disagrees: "Nimok havlad ad shelo yatza." In his view we cannot be certain that a child emerged together with the placenta, since it is possible that the fetus disintegrated and was absorbed within the mother's body and never came out at all. Accordingly, there would be no reason to declare the house tamei.
The halacha, however, does not follow Rebbi Shimon. We rule like the Tanna Kamma: wherever a placenta has emerged, we must presume that a child came with it.