A woman who gives birth is considered impure. If the newborn is male — she is impure for 40 days; if the newborn is female — she is impure for 80 days, as is mandated by the Torah (Leviticus chapter 12). Even if the woman miscarried and the fetus is not viable, the woman is still impure. But since women sometimes miscarry at the start of their pregnancies, and the fetus can take on various odd shapes, and since one cannot know with certainty what she miscarried — male, female, or something not human at all — the sages of the Mishnah found themselves in disagreement.
According to one sage, if she miscarried a lump in the form of an animal, beast, or a bird — she is impure; according to other sages she is pure as long as she has not miscarried a lump in the form of a person.
The Talmudic sages (the Amoraim) try to give their own explanation of the disagreement between the sages: according to the one who rules the animal-like lump is impurifying, it is because the Scriptures account their creation similar to that of Adam (both creations are termed yetzirah by the Scriptures. It speaks of the creation of Adam thus: “And G-d created [yatzar] Adam with dust from the earth” [Genesis 2:7] and of the creation of animals thus: “And G-d created [yatzar] from the earth all beats of the field and all birds of the skies” [Genesis 2:19]). The scholars asked: If so, if the language of creation determines law for the form of a miscarriage, then the mother is impure. We should also consider a woman impure if she miscarries a lump in the form of a crocodile, for it is written “And G-d created (bara) the crocodiles” and of Adam it is written “And G-d created (bara) Adam.” (The Talmud goes on at length explaining with great sophistry why a crocodile does not have the same rule as do the other animals.) The scholars also ask: If so, if the woman miscarries a mountain she should also be impure, for the Scripture also describes mountains with the same terminology; it is written “For behold, He creates (yotzer) mountains” (Amos 4:13).
They answered the scholars: a woman does not miscarry a mountain, only a rock. This rock is called a lump, not a mountain.

Other scholars explained why one who miscarries a lump in the form of an animal or bird is impure. They suggested that this is because animals have eyes similar to human eyes. If so, the question arises — if she miscarries a lump in the form of a snake she should be impure, for the snake’s eyes are like men’s eyes. The sages answered: it is so, and if a woman miscarries a lump in the form of a snake, she is impure.
Other scholars gave another explanation for why a woman is impure if she miscarries a lump in the form of an animal: Animals have eyes at the front of their faces and look forward, as do men. The scholars ask, if so, why a woman is impure if she miscarries a lump in the shape of a bird? A bird has eyes at the sides of its face?! They answered: The sage who impurifies the woman referred to an owl, whose eyes are at the front of its face and not on the sides, but the form of other birds, whose eyes are at the sides, would not impurify the woman who miscarries in their shapes.
(Babylonian Talmud, Tractate Niddah 22-23a)
The Talmudic text:
Mishnah: If an abortion had the shape of a beast, a wild animal or a bird, whether clean or unclean, if it was a male she must continue [in uncleanness and subsequent cleanness for the periods prescribed] for a male, and if it was a female she must continue [in uncleanness and subsequent cleanness for the periods prescribed] for a female, but if the sex is unknown she must continue [in uncleanness and subsequent cleanness for the periods prescribed] for both male and female; so Rabbi Meir. The sages, however, ruled: anything that does not have the shape of a human being cannot be regarded as a human child.
Gemara. Rav Yehudah citing Samuel stated: What is the reason of Rabbi Meir? Since in their case an expression of forming is used as in that of man. Now then, if an abortion was in the likeness of a sea-monster would its mother be unclean by reason of child-birth, since an expression of forming was used in its case as in that of man, it having been said, “And G-d created the great sea-monsters?” (Genesis 1)? I can answer: An expression of forming may be deduced from another expression of forming but one of creating may not be deduced from one of forming…
Rabbi Chiyya the son of Abba, citing Rabbi Yochanan, also stated, This is the reason of Rabbi Meir: Since the expression of ‘forming’ has been used in its case as in that of man. Said Rabbi Ammi to him: Now then, if an abortion was in the shape of a mountain would the woman who aborted it be unclean by reason of the birth because it is said, “For, lo, He that formed the mountains and created the wind?” (Amos 4) The other replied: Does she ever abort a mountain? She can only abort something in the shape of a stone, and that can only be described as a lump. But then, if the abortion was some inflated object would the woman who aborted it be unclean by reason of the birth because the expression of ‘creating’ has been used about it as about man, since it is written, “And created the wind”? And should you reply: it is not available for deduction, [it could be retorted:] Since it could have been written, ‘Formed the mountains and the wind’, and yet it was written ‘And created the wind’ it may be inferred, may it not, that it was intended to be made available for deduction? The other replied: An analogy for legal purposes may be drawn between words that occur in the Pentateuch, but no analogy may be drawn between words that occur respectively in the Pentateuch and in the post-Pentateuchal books. Ravah the son of Bar Chanah, citing Rabbi Yochanan, stated: This is the reason of Rabbi Meir: Because [the pupils of] their eyes are similar to those of human beings. Now then, if an abortion was in the likeness of a serpent would the woman who aborted it be unclean on account of the birth since its eye-ball is round like that of a human eye? And should you suggest that the law is so indeed [it could be retorted]: Why then was not the serpent mentioned? If the serpent had been mentioned it might have been presumed that only in the case of the serpent do the rabbis disagree with Rabbi Meir, since the expression of ‘forming’ was not written about it but that in the case of a beast or a wild animal they do not differ from him since the expression of ‘forming’ had been written about it. But was it not stated in regard to blemishes, ‘One whose eyeball is like that of a man’? This is no difficulty, the one refers to the black of the eye while the other refers to the slit.
Rabbi Yannai stated, This is the reason of Rabbi Meir: Because their eyes are fixed in the front of their heads like those of men. But what about a bird whose eyes are not fixed in the front of its head and Rabbi Meir nevertheless ruled that it is a cause of uncleanness? Abaye replied: This applies only to the kadia and the kipufa. It does not then apply to other birds!