Gemara

(Rav Nachman): If one acquires on behalf of a fetus, the fetus does not acquire. (If the giver said) 'when she will give birth...', he acquires.

(Rav Sheshes): In both cases, the baby acquires.

Support (Rav Sheshes for himself - Beraisa): If a convert died, and Yisre'elim seized his property, and they heard that he left a son, or that his wife was pregnant, they must return it. If after returning it they heard that the son died or that the wife miscarried, the first Kinyanim are void. The property is Hefker.

If a fetus does not acquire, the first Kinyanim should be valid!

Rejection #1 (Abaye): Inheritance is different, for it is automatic.

Rejection #2 (Rava): There, the initial Kinyanim were insecure (the people did not know whether or not there were heirs).

Question (Mishnah): A one-day old baby inherits and bequeaths.

Inference: Less than one day (i.e. a fetus) does not inherit!

Answer (Rav Sheshes): A one-day old baby inherits his mother's property to bequeath it to his paternal brothers, but a fetus does not.

This is because the fetus always dies before the mother. A dead son does not inherit his mother to pass the property to his paternal brothers.

Nidah 43b: A one-day old obligates his brother's widow to do Yibum (or Chalitzah), he exempts his mother from Yibum... he permits to eat (Terumah) and forbids to eat, he inherits and bequeaths...

44a (Rav Sheshes): The Mishnah discusses a Kohen with children from a permitted wife, and a child (a Chalal) from a forbidden wife. The Kohen died, leaving slaves to his children. The Chalal inherits a share in the slaves, and he forbids them to eat Terumah. Our Tana argues with R. Yosi, who says that even a fetus (partially owns the slaves, and) forbids.

Yevamos 67a (Mishnah - R. Yosi): If a Bas Yisrael was widowed from a Kohen and she was pregnant, her slaves do not eat Terumah because the fetus owns part of them. A fetus forbids eating, but does not permit;

Chachamim: If so, this should apply even if she is a Bas Kohen!

(Beraisa - R. Shimon): If he left sons, her slaves eat.

He is not concerned for the minority (that the fetus is a viable male).

Kesuvos 38b - Question (Rava): If a Na'arah died, does she become a Bogeres in the grave (six months after she became a Na'arah)?

If she became a Bogeres, if a fine was owed to her, she gets it and her child inherits it. If she didn't become a Bogeres, her father gets it.

Question: She cannot become pregnant (and give birth before becoming a Bogeres)!

Rishonim

Rif and Rosh (56a and 8:18): A fetus does not inherit its mother's property to bequeath it to his paternal brothers, for it dies before the mother.

Rambam (Hilchos Nachalos 1:13): If the mother died, and afterwards her son died, even if he died the day he was born and he was premature, since he lived for a moment, he inherits his mother and bequeaths to his paternal heirs.

Rebuttal (Ra'avad): This is wrong. He bequeaths only if he was born after a full term pregnancy. If not, his heir is Safek, and the baby's maternal brothers are Vadai, and a Safek cannot take from a Vadai.

Magid Mishneh: Perhaps the Rambam learns from the Gemara, which says that a fetus does not inherit. He infers that a born baby inherits, even if it was premature. This not a proof; this requires investigation.

Magid Mishneh (Hilchos Yibum 1:5): The Rambam discusses when the Simanim were finished.

Poskim

Shulchan Aruch (CM 276:5): If the mother died, and afterwards her son died, even if he died the day he was born and he was premature, since he lived for a moment, he inherits his mother and bequeaths to his paternal heirs.

Be'er ha'Golah (50): The Rambam says that this is even if he was premature. It seems that the Shulchan Aruch omitted this, for he holds like the Ra'avad. Still, he should have made this clear!

Rema: This is only if he was born. A fetus does not inherit his mother, who died while pregnant, to bequeath to his paternal heirs.

SMA (7): If the mother was killed, or the fetus began to leave, it can live after the mother dies. There is such a case in which a fetus inherits. The Mishnah taught that a baby inherits, for this is Pesika (always true). Tosfos says so. The Ir Shushan says that in every case it is a Nefel. This is wrong. The Rambam says that even if he was premature, if he lived for a moment, he inherits and bequeaths. The Magid Mishneh says that this is when his Simanim were finished,. If so, the same applies to a fetus.

Bach (8): It is not Pesika that a born baby inherits! Tosfos (Nidah 44b DH Ha) says that he inherits only if he lived 30 days according to Chachamim, or if we know that the pregnancy was full term according to R. Shimon ben Gamliel! This is when it died through an accident, but if it yawned and died, all agree that it is a Nefel! The Tur wrote Stam, like the Rosh, for he saw that the Rambam connotes that a one day old inherits even if it yawned and died. I say that one cannot take property from a Muchzak.

Chasam Sofer (EH 2 161): The Bach holds like the Ran, that even if the baby fell sick and died, it is considered a Nefel. The Pri Megadim toiled to explain the Shulchan Aruch simply, that any one-day old inherits, for we follow the majority of women, who deliver viable babies. It is stronger than a Stam majority; we rely on it to transfer money. Really, no one says so! It is a weak majority, and we do not rely on it to take money from a Muchzak.

Ketzos ha'Choshen (1): The Acharonim rejected the Rambam. Lechem Rav (188) says that one who inherits a Nefel cannot say 'I hold like the Rambam', for all Chachamim disagree. I say that the Gemara supports the Rambam. R. Efrayim asked, in Bava Basra, why didn't we ask from the Reisha of the Mishnah, which says that only a born child forbids slaves to eat Terumah? We could not answer like we answer the Seifa (a born baby inherits his mother to bequeath to his paternal brothers, but a fetus does not, for it dies before the mother)! The Ri answered that Rav Sheshes holds that Chachamim hold that a fetus can acquire, but they rely on the majority (half of all fetuses are females, and some are miscarried). The Gemara said that seizure of property before the convert's wife miscarried is invalid (even though the fetus was a Nefel)! Tosfos (Yevamos 67b DH Ein) answers that a fetus inherits until it is miscarried. Chachamim add the minority of fetuses that already died to show that the majority do not inherit. This is a proof for the Rambam, that a Nefel inherits. Why do Poskim say that the Rambam is a lone opinion? Tosfos agrees! The Ramban, Rashba, Nimukei Yosef, Magid Mishneh and Kesef Mishneh disagree. The Ramban dispels this proof by saying that 'Hepilah' means that the forehead came out, and afterwards the baby died. This is difficult, for Hepilah connotes that she miscarried. Also, if so what was the proof that a fetus inherits? Emergence of the forehead is full birth! The Nimukei Yosef (Yevamos 10b) says (according to Reish Lakish, whom the Halachah follows) that if a woman miscarried, this does not reveal retroactively that she was not properly pregnant. Perhaps the fetus was viable, and afterwards something caused a miscarriage! If so, we can say that since most women give birth to viable babies, we assume that when the first people seized, she was pregnant with a viable child.

Rebuttal (Nesivos ha'Mishpat 2): Perhaps Tosfos does not hold like the Rambam, rather, like the Nimukei Yosef! Rav Sheshes holds that a fetus inherits, for we assume that it is viable, but if a baby was born with unfinished Simanim, this shows that it is a Nefel and it does not inherit!

Question (Ketzos ha'Choshen): In Kesuvos, Rava asked about Bagrus after death. The Gemara asked that she cannot have a child to inherit her before Bagrus. If a Nefel inherits, what was the question?!

Answer (Nesivos ha'Mishpat 2): This is not difficult for the Rambam. The Magid Mishneh explained that the Rambam holds that a Nefel inherits for we assume that it was really a seven-month baby that was delayed. We cannot say so if it was born before six months! Also, perhaps a baby born before six months does not live at all.