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1. There is a dispute about whether the emergence of the head is considered a birth.
2. The Mishnah discusses cases in which there is uncertainty about the miscarriage.
3. The Mishnah is consistent with the law that a woman who miscarries over a river is presumed to have had only a miscarriage.
4. The Gemara discusses an animal that was pregnant, went to pasture, and came back after it clearly was no longer pregnant.
5. The Gemara discusses the Mishnah's (28a) statement that if most of the fetus emerged in pieces, it is considered a birth.
A BIT MORE
1. Rebbi Eliezer says that it is not, while Rebbi Yochanan says that it is. They disagree only about a child that does not live more than 30 days. Once most of the head of a healthy child emerges, it is considered born.
2. If the gender of the fetus is unclear, the mother observes the days of Taharah for a boy and the days of Tum'ah for a girl, out of doubt. If it is only an apparent miscarriage and it is ultimately unclear if there even was a fetus, the above law applies to the mother, and she also must suspect that she is a Nidah (with all of its implications).
3. The Mishnah is discussing a case in which the woman was not noticeably pregnant. If it is not clear that a fetus was present, she must also suspect that she is a Nidah. This is in contrast to the law of Rebbi Yehoshua ben Levi in the case of a woman who was noticeably pregnant, who does not have to suspect that she is a Nidah.
4. The Gemara says that the next birth of this animal has the status of a doubtful firstborn. In this case, the animal did not show signs of being soiled from birth (which most animals that give birth have). Moreover, it is possible that she miscarried something that would not exempt the next animal from having the status of a firstborn.
5. While this is clearly true in the case of a fetus that did not come out head-first, the Gemara quotes Tana'im who discuss how much of a fetus that comes out head-first must be born in order for the birth to be considered a Halachic birth.
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