Gemara

(Mishnah - R. Meir): If a man married Me'uberes (Chavero) or Meinekes Chavero (a woman pregnant with or nursing a baby from another man), she does not drink nor receive a Kesuvah;

Chachamim say, he can separate from her and remarry her (after the nursing period ends. Since she could be married to him later, she may drink.)

26a (Beraisa - R. Meir): One may not marry Me'uberes or Meinekes Chavero. If he married her, he must divorce her and may never remarry her;

Chachamim forbid marrying her. If he did, he must expel her. When the period of nursing finishes, he may remarry her.

Yevamos 36b (Mishnah): (If a Yevamah did Yibum and was found to be pregnant...) if she gave birth to a Nefel (they may remain married.)

(Beraisa - R. Eliezer): He divorces her with a Get.

(Rava): R. Meir and R. Eliezer hold similarly.

Objection (Abaye): If one transgressed Bi'ah with his brother's wife, an Isur mid'Oraisa, R. Eliezer forces him to divorce her. Perhaps he is lenient about Me'uberes Chavero, which is mid'Rabanan!

Perhaps R. Meir is stringent about a mid'Rabanan Isur, for Chachamim strengthened their words more than mid'Oraisa laws, but (we need not decree to make) people refrain from a mid'Oraisa transgression!

(Rava): Chachamim obligate one who married Me'uberes Chavero to divorce her.

Support (Mar Zutra): The Beraisa says 'he expels her', not 'he separates.'

37a (Rav Ashi) Question: A Kohen who married a Safek Shomeres Yavam may keep her.) If Levi married a woman pregnant from or nursing a baby from David, are we lenient for a Kohen (he need not divorce her, so he may remarry her after she finishes nursing)?

Answer (R. Hoshaya brei d'Rav Idi): No. Chachamim (who argue with R. Shimon ben Gamliel) assume that even if a baby died within 30 days, it was viable. A Kohen cannot keep his wife if she does Chalitzah, so we rely on Chachamim;

Here, we cannot rely on R. Meir (who says that Reuven must divorce her and may never remarry her), nor on Chachamim (who obligate divorcing her)!

If a man who was Mekadesh a woman within three months (after she was widowed or divorced) and fled, Rav Acha and Rafram argued about whether we excommunicate him (until he divorces her), or if it suffices that he fled.

A case occurred, and Rafram said that it suffices that he fled.

Kesuvos 60a: Abaye permitted a man to be Mekadesh a woman nursing a 15 month old baby, for several opinions permit even Nisu'in. Rav Yosef reminded him that Rav and Shmuel require waiting 24 months, excluding the days of birth and of Kidushin. Abaye ran to retract his ruling, but was unable to catch the man.

Rishonim

Rif and Rosh (Yevamos 10b and 4:4): One may not marry a pregnant or nursing woman. We rule like Chachamim; he must divorce her, even if he is a Kohen.

Question (Rosh): Why is it a support that the Beraisa says 'he expels her', and not 'he separates from her'? Regarding a Sotah Me'uberes Chavero, Chachamim also say 'separates', and surely it is with a Get, like here!

Answer #1 (Rosh): 'Separate' can mean with or without a Get. 'Expels' is only with a Get.

Answer #2 (Rosh): A Sotah is forbidden to him until she drinks, so we are not concerned lest they have Bi'ah, so he need not divorce her. R. Yosef of Orlins likewise permitted one who was Mekadesh Meinekes Chavero to separate without a Get, since an Arusah before Nisu'in is forbidden like a Nidah. Only one who made Nisu'in must divorce her. Abaye tried to retract his Heter to be Mekadesh, but we do not find that he forced the man to divorce her! We forbid a nursing woman to accept Kidushin or have Nisu'in. If she needed a Get after Kidushin, we would not need to say that Nisu'in is forbidden! One who was Mekadesh before three months (a widow or divorcee must wait before remarrying for Havchanah, to avoid doubts about from whom she is pregnant) must give a Get if he did not flee. Perhaps we are more stringent about Havchanah.

Rebuttal (Rosh): We are not concerned lest one transgress the Isur Sotah, which is mid'Oraisa. If the only Isur is Me'uberes Chavero, which is mid'Rabanan, we are concerned, and even if there is a second Isur mid'Rabanan of an Arusah! Rabbeinu Meir forces one who made Nisu'in with Me'uberes Chavero to divorce her, even if he vowed Al Da'as Rabim (on the will of others) not to benefit from her until she finishes nursing, which forbids her mid'Oraisa. Sotah is different; a man despises her for being secluded after he warned her. Perhaps Abaye forced the man to divorce her; the Sugya was not discussing that. Some texts say 'she may not have Nisu'in, or (even) accept Kidushin. Perhaps our text mentions Nisu'in because R. Yehudah permits even Nisu'in after 18 months.

Rambam (Hilchos Gerushin 11:28): If one transgressed and had Nisu'in with a pregnant or nursing woman in this time, he divorces her, even if he is a Kohen. If he is a Yisrael, he may remarry her after 24 months of nursing. If he married her and fled and later settled with her, this is fine. If one was Mekadesh a pregnant or nursing woman, we do not force him to divorce her or (allow) Nisu'in until after the period of nursing or until the baby dies.

Poskim

Rema (EH 13:11): Some say that if Leah gave her baby to a wet-nurse, who swore to nurse for the entire period, and Leah had Nisu'in, her husband need not divorce her.

Gra (39): Teshuvas Maimoniyos (24) permits. All other Poskim forbid.

Rivash (360, cited in Beis Yosef (DH u'Mah she'Chosav Rabeinu v'Im): We ceased to let widows swear to collect the Kesuvah (Gitin 35a), for they used to 'permit' to themselves matters and swear falsely. Perhaps the wet-nurse will do so. Here, one opinion was even concerned lest a woman kill her baby!

Beis Shmuel (25): It is not clear whether a security helps b'Di'eved if she had Nisu'in

Shulchan Aruch (12): If one transgressed and had Nisu'in with a pregnant or nursing woman within 24 months, we excommunicate him unless he fled. He divorces her, even if he is a Kohen. He must pay a Kesuvah if she claims from him. If he is a Yisrael, he may remarry her after 24 months of nursing. He writes another Kesuvah for her. If he married her and fled and afterwards settled with her, this is fine. If one was Mekadesh a pregnant or nursing woman, we do not force him to divorce her. He may not make Nisu'in until after the period of nursing or until the baby dies.

Rema: Some do not distinguish between Kidushin and Nisu'in. This is primary.

Rivash (ibid. DH v'Im): It seems that the Rambam does not make him divorce her, but we excommunicate him for transgressing a mid'Rabanan law. He holds that if we would force to divorce, it would not suffice if he fled. The Beraisa says 'if he made Nisu'in', he must divorce her. We have no source regarding Kidushin. Regarding Havchanah, even if he made Nisu'in he need not divorce her, for it is only three months. The Ra'avad, Ramban and Rashba obligate divorce in every case. The Beraisa mentions Nisu'in, for then even R. Yehudah agrees. The Halachah follows R. Meir's decree to force even an Arus. Rashi says that fleeing reveals that he does not plan to make Nisu'in until she finishes nursing, so we do not force him to divorce. If he showed intent to make Nisu'in, and only now fled, we excommunicate him until he divorces her. Perhaps fleeing suffices only for Havchanah, which is an Isur, but not for Meinekes Chavero, which is a danger.