Gemara

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 child inside you', this is a Kinyan on behalf of a fetus, which is invalid.

Question (Rav Nachman - 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 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.

Question: Why didn't he say that the Mishnah is when he said 'when she will give birth, the baby will receive...'

Answer: He holds that even in this case, the baby does not acquire.

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, it 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.

147b (Rav Nachman): If a Shechiv Mera said 'Ploni will live in this house or eat the fruits of this tree', it does not take effect. He must say 'give this house to Ploni and he will live in it, or this tree to Ploni and he will eat the fruits'.

He holds that Matnas Shechiv Mera takes effect only in those cases when a gift of a healthy person takes effect.

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.

(Rosh citing Ramban): We say that Zechiyah for one's son acquires, because one feels great affinity for his son. If Zechiyah works, it should always work! Rather, we discuss a Shechiv Mera. He acquires when he is born. Chachamim enacted this because one feels great affinity for his son. If a Shechiv Mera cannot do so, perhaps he will be upset (and die).

Rejection (Rosh): Chachamim enacted that a Shechiv Mera's words are as if a healthy person wrote and gave a document, but no more than this. The Halachah is, a fetus disqualifies (from Terumah), but not if there are brothers, like R. Shimon taught.

Defense (Shach CM 210:1): Initially, the Gemara held that Zechiyah for a fetus works, and all the more so inheritance. We conclude that Zechiyah for one's fetus works only because he loves him, and surely he acquired for him in a way that helps, e.g. 'acquire from now on condition to be Makneh (later, to my son).' Even if no way helps, Chachamim enacted that he acquires, for one loves his son.

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.

Poskim

Shulchan Aruch (CM 210:1): If one was Mezakeh for his fetus, he acquires, even if he did not say 'when it will be born.'

Gra (6): Tosfos (Gitin 13b DH l'Davar) and Hagahos Ashri (9:5, b'Sof) hold that Rav Nachman does not say that he must say 'when it will be born.' Rather, he teaches that the Kinyan is only from when it will be born, i.e. until then he can retract.

Gra (7): The Gemara said 'why didn't he say that in the Mishnah, he said 'when she will give birth'?' This shows that Stam, we understand that he did not say this, yet he acquires.

Mabit (1:339, cited by R. Akiva Eiger): The enactment was only for a father to his son. A mother also loves her son, but it is not common for her to give such a gift. Alternatively, the Torah knows that a man gives his money to his son more freely, for he himself can work and earn more. It says "k'Rachem Av Al Banim", and "merciful women cooked their children."

Rebuttal (Devar Moshe 69): No Posek says so. The Mabit (3:137) also says that just like a father can acquire to his son who is not yet in the world, he can acquire to him Davar she'Lo Ba l'Olam. No Posek agrees.

Shulchan Aruch (ibid): This is if his wife was pregnant at the time of the gift.

Beis Yosef (DH v'Im): The Ba'al ha'Itur says that Zechiyah for one's own fetus helps even within 40 days, as long as the fetus is evident. This seems correct. The Nimukei Yosef says that it does not help within 40 days, for the fetus is considered mere water then.

Shach (2): The Nimukei Yosef's opinion is primary. A fetus within 40 days does not disqualify slaves from Terumah. The Rambam (Hilchos Terumos 8:3) says so. There is no proof from the Tur, for within 40 days it is not called a fetus.

She'eris Yosef (12, cited by R. Akiva Eiger): When he says 'when she will give birth', the baby acquires, for the Kinyan is only later, when he is in the world. Therefore, it is even if his wife was not pregnant at the time! The Tur distinguishes between whether or not she is pregnant only regarding a gift to one's son, but not regarding a gift 'when she will give birth.'

Rema: A grandson is like a stranger.

SMA (3): One does not feel so close to a grandson. Similarly, the Kinyan works only if the baby is viable.

Question (Shach 257:3): Regarding one who wrote his property to his son 'after my death', the Rema (257:2) says that a grandson is like a son (and receives attached Peros)! One can resolve this with difficulty.

Shulchan Aruch (ibid): Some say that only a Shechiv Mera can acquire for his son, but a healthy man cannot.

Beis Yosef (DH Mihu): The Ramban proves from the Yerushalmi that a Shechiv Mera can acquire for his son, for he loves him and we are concerned lest his mind falter (if he could not acquire for him). The Rif and Ran say the same. The Rashba (1:933) says that he must be a Shechiv Mera so it will not be Asmachta. The Tur holds that it is because coins cannot be acquired through Chalipin. The Rambam holds that he need not be a Shechiv Mera.

Shach (1): The Rif means that the Kinyan works through words alone only for a Shechiv Mera. A healthy person must do a Kinyan.

Gra (10): The Ramban holds that the Halachah does not follow R. Yosi, and a fetus does not inherit, so all the more so Zechiyah for a fetus does not work, except for Shechiv Mera, which is an enactment. Tosfos learns otherwise from 141b (if one told his pregnant wife...) The Yerushalmi says the opinion that it works only for his son says so only for a Shechiv Mera.

Shach (1): The Rif, Rambam and most Poskim, hold that even a healthy man can acquire for his son. The Yerushalmi says that only a Shechiv Mera can acquire for his son, and only Metaltelim. The Bavli does not say so. It holds that anyone can acquire for his son. No Poskim limit this to Metaltelim.