Gemara

(R. Yochanan): If one was Makdish a pregnant animal for a Chatas and it gave birth, he may bring either animal for his Chatas.

He holds that one can be Meshayer (be Makdish a pregnant animal, and stipulate that the fetus remain Chulin), for Ubar Lav Yerech Imo (a fetus is not considered like a limb of the mother, rather, like an independent animal). It is as if he was Makdish two Chata'os for Acharayus. He may offer either for Kaparah.

Question (Beraisa): If one (gave a document of freedom, and) said to his pregnant Shifchah "you are (still) a slave, and your fetus is free", she acquires freedom on behalf of her child.

Granted, if Ubar Yerech Imo, it is as if he frees half the slave. R. Meir holds that this works.

However, if one Ubar Lav Yerech Imo, how can she acquire for her child? A Beraisa teaches that a slave cannot acquire freedom for another slave (in this case, the child) of his own master!

Conclusion: One cannot be Meshayer. R. Yochanan is refuted.

Suggestion: Tana'im argue about this.

(Beraisa - R. Yosi ha'Gelili): If one said to his slave "you are free, and your fetus is a slave", her fetus is (free) like her;

Chachamim say, his words takes effect. It says "ha'Ishah vi'Yladeha Tihyeh la'Adoneha."

(Rava): Really, R. Yosi brought the verse for his law. Her children belong to her master only when she belongs to him.

Suggestion: R. Yosi holds that one cannot be Meshayer. Chachamim hold that one can.

Rejection: No, all hold that one can be Meshayer (regarding Korbanos). R. Yosi learns from the verse that freeing the mother frees her children.

Rishonim

Rambam (Hilchos Avadim 7:5): If one wrote to his pregnant Shifchah 'you are free and your fetus is a slave', his words are fulfilled. If he wrote "you are (still) a slave, and your fetus is free", his words have no effect, for this is like freeing half of her.

Rebuttal (Ra'avad): The Reisha contradicts the Seifa! The Seifa holds that Ubar Yerech Imo. If so, why are his words fulfilled in the Reisha?! Rather, in the Reisha his words are fulfilled because Ubar Lav Yerech Imo. In the Seifa his words have no effect because it is Lo Ba l'Olam, or because a slave cannot receive a Get Shichrur for another slave of his master.

Radvaz (1:188): Migdal Oz was satisfied with this answer for the Rambam. This cannot be, for the Rambam explained that it is like freeing half of her! Also, the Rambam holds that Ubar Yerech Imo! Rather, he holds that Ubar Yerech Imo. The fetus is part of the mother, but the mother is not part of the fetus. The Reisha is because all agree that if one was Meshayer, it is excluded, like R. Yochanan said. R. Yosi ha'Glili learns from a verse. It is a Gezeras ha'Kasuv regarding a Shifchah, but not because Ubar Yerech Imo.

Question: These depend on each other! The opinion that Ubar Yerech Imo holds that one cannot be Meshayer, and the opinion that Ubar Lav Yerech Imo holds that one can be Meshayer.

Answer (Radvaz): Indeed, Rashi and the Ra'avad hold like this, but the Rambam holds that they do not depend on each other. Always, the fetus is Nigrar after (it gets the status of) its mother, but not vice-versa. Therefore, when he says 'you are free and your fetus is a slave', it would be proper to say that Ubar Yerech Imo, and the child is (free) like her. However, we hold that one can be Meshayer. Therefore, his words are fulfilled, since he explicitly stipulated that the child is a slave. This is unlike freeing half a slave, for the mother is not Nigrar after her child. However, if he said "you are a slave, and your fetus is free", Ubar Yerech Imo and the fetus is Nigrar after the mother. This is like freeing half of her. We hold that this does nothing.

Radvaz: Since the Rambam holds that Ubar Yerech Imo, and the Ra'avad agreed, just he asked that the Reisha contradicts the Seifa, surely if one freed a Shifchah Stam, even if he did not know that she is pregnant, the child is like her. Even the one who says that Ubar Lav Yerech Imo agrees that the child follows her. He argues only about one who stipulated. However, how can we explain if Ubar Lav Yerech Imo and he did not know that she is pregnant? The child is Nigrar after his mother, unless one specified. Also, people free slaves generously. Also, he does not want sin to come through him, i.e. the child will marry a Yisrael (unaware that he is a slave)! If this were not true, the Tana should have taught that if one freed his Shifchah and she was found to be pregnant, the child is a slave. Do not say that he prefers to teach the extremity of R. Yosi's opinion, that even when he specified that the child will be a slave (the child is free). Firstly, since it is a Gezeras ha'Kasuv, it does not matter whether or not he specified. The children always follow the mother. Also, since the Rambam rules like Rabanan, it is better to teach the extremity of Rabanan. Rather, the Tana'im argue only about one who stipulated. Stam, the child is like her.

Rambam (Hilchos Mechirah 27:7): If one said 'I sell to you a pregnant slave or cow', he sold the child with her. If he said 'I sell to you a nursing slave or cow', he did not sell the child with her.

Radvaz (ibid.): This is a Tosefta. It seems that the child is included (in the Reisha) only because he mentioned that she is pregnant, but Stam, the child is not sold. This is wrong. Since she is pregnant, he needed to stipulate 'except for the fetus', since Ubar Yerech Imo. Even according to the opinion that Ubar Lav Yerech Imo, the fetus pertains to its mother and it is Nigrar after it. If he did not sell it, he should have specified. Rather, even Stam, he sold the child. The Tosefta discusses 'a pregnant Shifchah', to imply that when he sold 'a nursing Shifchah', even if she is pregnant, he did not sell the child. The general rule is, if one did not specify, everything is sold. Surely according to the opinion that Ubar Yerech Imo, everything is sold, and there is no Ona'ah or mistake. A pregnant Shifchah (without the fetus) is worth less than a Stam Shifchah, due to the danger of birth, and she serves less. Also, we hold that people sell generously. All the more so, one who freed his slave freed also the fetus. We hold that one who frees a slave transgresses an Aseh. Surely, he intended to permit her to marry Yisre'elim, and all the more so (he intended to permit also the child).

Note: This is not clear to me. There are many other reasons why one might free a Shifchah - he wants to marry her (Gitin 40a), he feels gratitude to her (Tosfos Gitin 12a DH Kol), others paid to redeem her (Kidushin 22b), she is not worth the cost of feeding her (Gitin 12a), or she is a thief or a bad influence on his household...!

Radvaz: This is like the Beraisa. R. Yosi ha'Glili and Chachamim argue only when he explicitly left over, but if he said Stam to a Shifchah (you are free), Chachamim agree that her child is like her. The Beraisa connotes like this, for it says 'his words are fulfilled', and not 'the child is a slave.'

Poskim

Shulchan Aruch (YD 267:61): If one wrote to his pregnant Shifchah 'you are free and your fetus is a slave', his words are fulfilled. If he wrote "you are a slave, and your fetus is free", his words have no effect.

Prishah (107) and Shach (75): In the Seifa his words have no effect because Ubar Yerech Imo. The fetus is like half the slave. The Shifchah is not Batel to the fetus. When he says 'you are free and your fetus is a slave', this is unlike half. The fetus is Batel to its mother; it is Nigrar after its mother, but its mother is not Nigrar after it. The rule is, if she remains a Shifchah, how can the fetus be free? It cannot exist without her. It nurtures from and lives from her. It is Batel to her; it is like her flesh. It is no better than freeing half of her. When he frees her, she can live without the fetus. This is not like freeing half a slave. Rather, it is as if the fetus was born, and is not attached to her at all, so it remains a slave.

Rebuttal (Taz 30): Do not say that sometimes Ubar Yerech Imo, and sometimes not. The Tur explained that in the Seifa his words have no effect because this is like freeing half of her. If so, the Reisha is difficult, for this connotes that he holds that in the Seifa, Ubar Yerech Imo. I answer that in the Reisha we explain his words in the most reasonable way, i.e. you will be free when you give birth. Surely, as long as the fetus is inside her and he is a slave, she is not free. Rather, he discusses after birth. The freedom begins now, and finishes with birth, when the child is not Yerech Imo. In the Seifa, we cannot say that the fetus will be free when it is born, for we hold (CM 210:1) that one who acquires on behalf of a fetus does not acquire. Rather, we must say that he acquires for the fetus now, through his mother. It helps for him Agav (along with) the mother, since it is part of her. If so, he frees half of her.