Gemara

142a (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 Huna): In both cases, the baby does not acquire.

(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.

They argue in a case when people heard that he died, and later that he had not died, and later (that) he really died.

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.

141a - Question (against Rav Huna - Mishnah): If a man said 'if my wife will give birth to a male, he should receive 100 Zuz', we fulfill his words.

Rav Huna: I cannot explain this. No Tana holds like this!

Question: Why didn't he say that the Mishnah is R. Yosi?

(Mishnah - R. Yosi): A fetus (a Kohen's son) disqualifies slaves from eating Terumah (for he owns a share of them).

Answer: Inheritance is different than Kinyan, for it is automatic.

Yevamos 67a (Shmuel): This is R. Yosi's opinion. Chachamim say, if the Kohen left sons, her slaves eat due to his sons. If he left no sons, they eat due to his brothers. If he left no brothers, they eat due to the whole family.

Shmuel taught that Zechiyah for a fetus is valid (like R. Yosi).

142b (R. Yochanan): If one acquires for a fetus, it does not acquire.

The Mishnah says that the fetus acquires, i.e. when one gives to his son. Since he feels great affinity for his son, he resolves to give.

Rishonim

Rif and Rosh (Yevamos 22b and 7:4): The Halachah follows Chachamim of R. Yosi, who say that a fetus does not acquire until it is born.

Question (Rosh citing Ra'avad): We hold like R. Yochanan, who says that if one acquires for his son, even in the womb, his son acquires, because one feels great affinity for his son. The Gemara (141b) said that inheritance, which is automatic, works better than Zechiyah. If so, all the more so a fetus inherits! If so, how can we rule against R. Yosi?

Answer (Rosh citing Ramban): The Halachah follows Chachamim against an individual (R. Yosi), even regarding inheritance, which is automatic. Even Shmuel, who holds like R. Yosi, said 'but Chachamim say...' Shmuel holds like R. Yosi because Shmuel holds that Zechiyah for a fetus works. The Halachah is unlike this, even regarding inheritance. Amora'im argued about Zechiyah for a fetus, and Rav Sheshes brought a proof from people who seized property of a convert who died. Abaye rejected the proof, for inheritance, which is automatic, is different. Rava rejected it because the initial Kinyanim were insecure. Abaye holds that all agree that a fetus inherits. Rava holds that Rav Nachman and Rav Huna hold that a fetus does not inherit. Had the initial Chazakah been solid, and then the Yisre'elim heard that he did not die, and he died, according to Rava the first Kinyan was valid. The Halachah follows Rava. We challenged Rav Sheshes from a Mishnah that says that a fetus does not inherit. Rav Sheshes answered according to his reasoning that it is because the fetus died first. This implies that Rav Nachman and Rav Huna hold that a fetus does not inherit.

Rosh: I say that Rava agrees that inheritance is different. Zechiyah does not work because the fetus in not in the world, so one does not resolve to give, except for to one's son, for whom he feels great affinity. If not, why didn't the Amora'im argue about whether or not a fetus inherits his father?

Question (Korban Nesanel 60): Perhaps they argue about Zechiyah to teach the extremity of the opinion that Zechiyah works!

Rosh: All Amora'im except for Shmuel hold that Chachamim agree that a fetus has Zechiyah. Rav Nachman and Rav Huna distinguish between a gift and an inheritance, and Rav Sheshes does not. Rava also distinguishes; he rejects Rav Sheshes' proof even according to Rav Sheshes, who does not distinguish. Rav Sheshes held that the Beraisa is even like Chachamim. They argue about Terumah, for they are not concerned for a minority. (Most fetuses do not inherit, for half are females, and some are stillborn.) Surely Rava holds like this, for he would not try to prove that R. Yosi holds that a fetus has no Zechiyah!

Rambam (Hilchos Zechiyah 2:18): If a convert died, and Yisre'elim seized his property, and they heard that he did not die yet, or left a son, or that his wife was pregnant, they must return it. If after returning it they heard that he or his son had really died, or that his wife miscarried, the first Kinyanim are void. Whoever takes the property afterwards keeps it.

Magid Mishneh: The Rambam holds that later they found out that the wife had miscarried before they seized. Abaye and Rava argue about when people heard that the fetus died, seized property, and it turned out that the fetus died only after they seized. According to Rava, since they heard that she miscarried, and the fetus has no Zechiyah, the first Kinyanim were valid.

Lechem Mishneh: According to Rava, why must they return? The fetus does not acquire, even if it was born later! Why didn't the Rambam bring the difference between Abaye and Rava? Rava holds that Zechiyah for a fetus 'when it will be born' works. He merely dispels Rav Sheshes' proof that Zechiyah works even without saying 'when it will be born'.

Rosh (Bava Basra 9:6): The Halachah follows Rav Nachman in monetary laws. Zechiyah for a fetus does not acquire, but inheritance, which is automatic, acquires. The Rambam rules like R. Yosi, that a fetus inherits.

Question (Beis Yosef CM 210 DH v'Chosav): What is the source that the Rambam rules like R. Yosi? In Hilchos Terumos (8:4), he says that a Chalal does not disqualify slaves, like Chachamim!

Hagahos ha'Bach (6): The text of the Rosh should say 'the Rivam rules like R. Yosi.

Poskim

Shulchan Aruch (CM 275:30): If a convert died, and Yisre'elim seized his property, and they heard that he did not die yet, or left a son, or that his wife was pregnant, they must return it. If after returning it they heard that he or his son had really died, or that his wife miscarried, the first Kinyanim are void. Whoever takes the property afterwards keeps it.

Beis Yosef (DH Ger): The Rashbam explains that the convert (or his son or the fetus) died after the Yisre'elim returned the property. The Rambam explains like the Tur, that after they returned they heard that he had already died. When they heard about the mistake, they withdrew from the property and their Chazakah was uprooted.

Gra (29): The Rambam explains that at the end, they heard that the first report was true. Abaye holds that they acquired only if there was no child or fetus. The Beraisa discusses when they heard that the convert had not died. (This is the Rambam's text, unlike our text which says that the son or fetus had not died.) The same applies in the other clauses if they heard that the son or fetus had not died. Hagahos Ashri says that Rivam (and the Rambam) rule like Abaye. Why didn't the Rambam explain the difference between Abaye and Rava? Really, they do not argue. The Gemara needed to say that the fetus always dies before the mother, for otherwise the fetus would inherit. One could reject this and say that Rav Sheshes said so (but we need not say so according to other Amora'im).