Commentary for Niddah 3:4

סנדל – a piece of flesh made like the shape of the tongue of a bull. And because it has the form of a sandal, we call it a sandal. And it customarily comes with the fetus. But there are those who explain, "סנדל" that it is becoming [as] a fetus [one must say, and poor}.

If she aborted a sandal or a placenta she sits in uncleanness for both male and female.
If a placenta is in a house, the house is unclean, not because a placenta is a fetus but because generally there can be no placenta without a fetus.
Rabbi Shimon says: the child might have been mashed before it came out.
Section one: A "sandal" is a fetus whose face has not yet been formed. This counts as a human miscarriage and if the sex cannot be determined, then she must observe the impurity rituals for having both a male and a female (see mishnah two).
While a placenta is not a fetus, it is a sign that there was a fetus. Therefore, since we can't determine the sex of the fetus, she must act as if she had both a boy and a girl.
Section two: If a placenta is found in a house, we can assume that a dead fetus was in the house as well. This makes the house into an "ohel" (a tent) which conveys impurity to all of the contents of the house. (We learned plenty about this in Tractate Ohalot).
Rabbi Shimon notes that the presence of a placenta is not a certain sign that she gave birth to a fetus. It is possible that the fetus could have been dissolved or mashed up (I realize that this is not pleasant) before it came out of the woman. In such a case, the fetus does not have the status of a dead body and doesn't defile as does a dead body. The house and its contents will remain pure.

שליא – like a skin/membrane that was fetus is lying within it. And its beginning is like the thread of woof [which is thicker than that of the warp – see Tractate Niddah 35a] (i.e., the latitudinal direction), but when it expands wider like a lupine and hollowed like a trumpet. But the placenta is not less than a handbreadth.

הבית טמא – because of the tent of the deceased, for a [human] fetus was in it and died.

נימוק הולד (the fetus was mashed) – and the it became blood and was combined with the blood of birth and was nullified in the majority. But the house is not impure. But the Halakha is not according to Rabbi Shimon.