Before us is Mishnah 1:5, the third in the series of Mishnayos dealing with miscarriages. This Mishnah lists women who have undergone some form of miscarriage, but what they miscarried is not considered a fetus at all, and therefore they are not obligated to bring the sacrifice that a woman brings upon giving birth. Similarly (although this is not explicitly stated in the Mishnah), these are the situations that are not considered an opening of the womb, meaning: the child born after them will be considered a firstborn if it is a male.
Miscarrying an unformed sac:
This refers to a woman who miscarried and what emerged from her is a sac, a sort of membrane - unlike the formed sac mentioned earlier in Mishnah 3, which was like a developing fetus, containing something like flesh and taking shape. Here there are three examples of an unformed sac, where the contents inside the membrane (which was supposed to become a fetus) did not take shape at all:
"Malei mayim" - The content inside that membrane is merely a clear liquid.
"Malei dam" - The membrane is filled with blood.
"Malei geninim" - Some explain this as being related to the word for various colors and hues, meaning a mixture that includes blood, flesh and bone; while others explain that this refers to small shreds of flesh that look like worms and maggots, small pieces and fragments of flesh inside the membrane.
In all of these cases, it is not considered a fetus.
A sac that resembles other creatures:
We learned earlier in Mishnah 3 that if a woman miscarries and the fetus bears no resemblance to a human at all, the Sages say that it is not considered a birth. However, Rabbi Meir said that if it resembles a wild animal, a domesticated animal or a bird, she is considered to have given birth and brings sacrifices, because the term for 'formation' is stated twice: the verb 'yatzar' (formed) stated regarding wild animals and birds is the same verb used regarding the formation of man.
The novel teaching of our Mishnah is that even Rabbi Meir concedes that if the resemblance is not to a wild animal, a domesticated animal or a bird, but rather to creatures of a different kind, it is not considered a birth:
Fish and water creatures.
Locusts.
Small teeming and swarming creatures.
Crawling insects - these too are a type of swarming creature, like those that grow on corpses and the like, maggots and worms.
These creatures bear no resemblance to a human being at all, and therefore if the miscarriage looks like this, like small pieces of clotted blood, it is not considered a birth and she does not bring a sacrifice for it.
Miscarrying on the fortieth day:
A woman who knows the time of her conception and miscarried on the fortieth day of her pregnancy, and certainly if it was less than forty days - this is not considered a fetus. The miscarriage must occur from the forty-first day onward, since only after forty full days is the matter considered an actual fetus, meaning that it has taken the shape of a fetus.
"Yotzei dofen":
Its literal meaning is one who was extracted through the walls of the abdomen, which is what is known today as a Caesarean section. When a baby is extracted from the mother's abdomen rather than through a regular birth, this is not halachically considered a birth. The reason for this is as follows: The verse states, "Isha ki tazria veyaledah" - this implies that it is not considered a birth unless she gives birth from the same place where she conceived. However, if the baby emerges from the side, such as being surgically extracted from her abdomen, this is not considered a birth, and she is not obligated to bring a sacrifice or anything similar.
Rabbi Shimon disagrees and says that even a woman who gives birth via a Caesarean section is considered to have given birth, and she must bring a sacrifice and follow similar laws. His source is based on an exposition of the word "o" (or) stated in another verse. It is an extra word, and from it he derives that there are two forms of birth - the regular way or through the side of the abdomen. However, the halachah does not follow Rabbi Shimon, and therefore, a yotzei dofen is not considered a birth and a sacrifice is not brought for it.
In summary: This Mishnah lists the types of miscarriages that do not obligate a woman to bring a sacrifice and do not exempt the womb regarding the laws of a firstborn: an unformed sac - whether it is full of water, full of blood, or full of fleshy matter; a fetus resembling fish, locusts, insects, or creeping things, regarding which even Rabbi Meir concedes; a woman who miscarries on the fortieth day or earlier; and a yotzei dofen, which according to the Sages is not considered a birth, and although Rabbi Shimon disagrees - the halachah does not follow his opinion.