Gemara

131a (Rav Papa): Even R. Meir says only that one can be Makneh (transfer ownership to) Davar she'Lo Ba l'Olam (something not here now), but he must be to Davar she'Ba l'Olam!

140b (Mishnah): If one said 'if my wife will give birth to a male, he should receive 100 Zuz', we fulfill his words;

141b (Rav Huna): If one told his pregnant wife 'my property is to the one inside you', a Kinyan on behalf of a fetus is invalid.

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

Rav Huna: I cannot explain our Mishnah. I do not know a Tana who holds like this!

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

(Mishnah - R. Yosi): A fetus disqualifies slaves (that he will inherit from his father, a Kohen) from eating Terumah.

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

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.

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

Question: The Mishnah says that the fetus acquires!

Answer: The Mishnah discusses a man giving to his son. Since he feels great affinity for his son, he resolves to give, so the Kinyan works.

The Halachah is, Zechiyah for a fetus does not acquire.

Rishonim

Rif (66a): The Halachah is, Zechiyah for a fetus does not acquire.

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

Rambam (Hilchos Mechirah 22:10): Zechiyah for a fetus does not acquire. If one acquires for his own son, since he feels great affinity for his son, he acquires.

Rosh (Bava Basra 9:5): The Halachah follows Rav Nachman in monetary laws. If one acquires on behalf of a fetus, the fetus does not acquire. (If the giver said) 'when she will give birth...', the baby acquires. This is if the giver did not die or retract until it was born. If he retracted or died, and the property fell to the orphans, Rav Nachman agrees that he did not acquire. The Kinyan is not finished until it is born. This is when he did not acquire through Kinyan Sudar (Chalipin with a garment), for when it is born, the Sudar returned to its owner (and the Kinyan lapsed). Also, we do not discuss Chazakah. Rather, Meshichah was done, and it is still in the Reshus of the Zocheh (the one who acquired for it) when he is born, or in the swamp, or land was given through a document and the Zocheh has the document when he is born.

Hagahos Ashri: When the giver is healthy or a Shechiv Mera, the fetus acquires only if he said 'when it will be born.' However, if one sold dates that will grow or a calf that will be born, (Ri; perhaps this should say 'Rav Nachman') holds that the seller can retract, for it is harder to be Makneh Davar she'Lo Ba l'Olam than to Davar she'Lo Ba l'Olam, opposite to Rav Huna. The Rashbam holds that Rav Nachman holds that regarding this, a fetus is like Davar she'Ba l'Olam.

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.

Poskim

Shulchan Aruch (CM 210:1): One cannot be Makneh to something not in the world. Therefore, if one was acquired for another's fetus, it did not acquire, even if he said 'it should acquire when it will be born.'

Beis Yosef (DH k'Shem): The Rashbam explains that Rav Nachman holds that if he said 'when it will be born', since there is no Kinyan until it is born, this is not like Zechiyah for a fetus (which does not acquire). We conclude that Zechiyah for a fetus does not acquire. We must investigate whether we rule like R. Yochanan, or like Rav Nachman. If it is like Rav Nachman, when he says 'when it will be born', he acquires. The Rosh and R. Yerucham rule like Rav Nachman. The Rif and Rambam did not mention Rav Nachman. Theh hold that we conclude that Zechiyah for a fetus does not acquire because R. Yochanan says so, not because Rav Nachman says so. R. Yochanan did not mention that if he said 'when it will be born', he acquires. This shows that he does not distinguish, so also we do not distinguish. The Ran proves this from R. Yochanan's need to say that the Mishnah is because one feels great affinity for his son. He did not say that the case is, he said 'when it will be born'! This is not a proof. Perhaps R. Yochanan holds that regarding his son, he acquires; the Mishnah discusses his son! The Rashbam holds like this. He says that the Halachah follows R. Yochanan, and if one acquires for a fetus that is not his, he does not acquire.

Gra (2): The Shulchan Aruch rules like R. Yochanan. The Gemara (131a and Gitin 13b) says that even R. Meir says only that one can be Makneh Davar she'Lo Ba l'Olam, but he must be to Davar she'Ba l'Olam. Tosfos (13b DH l'Davar) says that this is like Rav Huna; Rav Nachman says oppositely! The Rashbam rules like Rav Nachman, for he holds that a fetus is like Davar she'Ba l'Olam.

SMA (1): This is like Davar she'Lo Ba l'Olam, so he can retract even after the baby was born. The opposing opinion in the Rema holds that Adam Makneh Davar she'Lo Ba l'Olam. If one was Makneh Davar she'Lo Ba l'Olam, we do not take from one who seized. It seems that the same applies here.

Rebuttal (Taz): The opposing opinion is the Rosh, who rules like Rav Nachman, that when he said 'when it will be born', he acquires. The Tur (212:9) brings from the Rosh (Bava Kama 4:3) that if one said 'when I get this debt, it will be Tzedakah', he is obligated due to the power of Tzedakah. This implies that for commoners, Ein Adam Makneh Davar she'Lo Ba l'Olam! We must resolve Rav Nachman, for regarding dates that will grow, he allows retracting even after they grew! We must say like the Rashbam, that regarding his son it is as if it is already in the world. Also, the property is in the world.

Rema: This is unlike those who say that if the giver did not die or retract until he was born, he acquires. I.e. if he acquired through Meshichah, and it is still in the Reshus of the Zocheh (the one who acquired for it) when he is born, he acquires if the giver said 'when it will be born.'

Gra (4): Rav Huna holds that when one acquires Davar she'Lo Ba l'Olam, he can retract before it comes. Tosfos (Yevamos 93b DH v'Rav) says that Rav Nachman holds about l'Davar she'Lo Ba l'Olam like Rav Huna says about Davar she'Lo Ba l'Olam.

Tumim (61:1, cited by Gilyon R. Akiva Eiger): The Rosh rules like Rav Nachman. Therefore, since we hold that one can give a lien on Davar she'Lo Ba l'Olam, all the more so one can give a lien to Davar she'Lo Ba l'Olam. The Rashba rules like Rav Huna, therefore, one can give a lien only to one who was born already.

Prishah (DH u'Mah): Even if he made a fence and the fence is still standing when the baby is born, since the Kinyan did not take effect when he made the fence, later it does not take effect retroactively.