משנה אויף ויקרא 12:2

The following do not bring a sacrifice:A woman who discharges a sac filled with water or with blood or with pieces of flesh; Or if the miscarriage was in the shape of fish, locust, unclean animals or reptiles; Or if the miscarriage took place on the fortieth day [after the conception], Or if it was extracted by means of a caesarean section. Rabbi Shimon declares her liable [to an offering] in the case of a caesarean section.

Rabbi Akiba declared three things; about two they agreed with him, and about one they disagreed with him.About a lime-burner’s sandal, that it is liable to contract midras impurity; And about the remains of a [broken] oven, that they must be four handbreadths high [in order to retain impurity], whereas they used to say three and [when he said four] they agreed with him. And about one they disagreed with him About a stool, from which two of its covering-boards had been removed, the one beside the other, which Rabbi Akiba pronounces able to contract impurity, but the Sages declare unable to contract impurity.

If a woman miscarried an object that was like a rind, like a hair, like earth, like red flies, let her put it in water: If it dissolves she is unclean, But if it does not she is clean. If she miscarried an object in the shape of fishes, locusts, or any forbidden things or creeping things: If there was blood with them she is unclean, If not, she is clean. If she miscarried an object in the shape of a beast, a wild animal or a bird, whether clean or unclean: If it was a male she sits in uncleanness as she would for a male; And if it was a female she sits in uncleanness as she would for a female. But if the sex is unknown she sits in uncleanness for both male and female, the words of Rabbi Meir. The sages say: anything that has not the shape of a human being cannot be regarded as a human child.