Gemara

(Mishnah): If a man married a woman and she did not give birth in 10 years, he may not neglect Peru u'Rvu.

(Beraisa): If a man married a woman and she did not give birth in 10 years, he must divorce her and pay a Kesuvah. Perhaps (she is not sterile, just) he did not merit to have children through her;

64b - Question: He pays a Kesuvah because he did not merit to have children through her. Perhaps she did not merit conceiving from him!

Answer: Since she is not commanded about Peru u'Rvu, she would not be punished by inability to fulfill it.

(Mishnah): If he divorced her, she may remarry...

Inference: She may marry a second man, but not a third. Our Mishnah is like Rebbi:

(Beraisa #1 - Rebbi): If a woman circumcised two of her sons and they died, she should not circumcise her third son;

R. Shimon ben Gamliel says, she circumcises the third, but not a fourth.

Rava: Regarding marriage, the Stam Mishnah (and the Halachah) is like Rebbi.

65a (Beraisa): If a woman was married twice (each time for 10 years) and did not have children, afterwards she may marry only a man who has children;

If she married a man without children, she leaves him without a Kesuvah.

Question: If she did not have children from the third husband, can her first two husbands make her return the Kesuvos that she collected from them?

Can they say that now it is revealed that it is she who was barren?

Or, can she say that only now she became weak?

Answer: Presumably, the latter is true.

Question: If she married a fourth husband and had children, may she demand a Kesuvah from the third husband?

Answer #1: No. We tell her, it is better for you to be quiet, lest your third husband say that had he known (that you are fertile) he would not have divorced you (and you are still married to him)!

Objection (Rav Papa): If she keeps quiet, do we keep quiet? The divorce was void, and her children are Mamzerim!

Answer #2 (Rav Papa): Rather, we say that now she got better.

(R. Ami): If each of them says that the other is sterile, she is believed (and receives a Kesuvah). This is because she can tell whether or not he is Yoreh k'Chetz (his semen shoots out at the time of Bi'ah), but he cannot.

65b - A woman (to R. Ami): Force my husband to (divorce me and) pay my Kesuvah (we are childless).

R. Ami: No, you are not commanded to have children!

The woman: What will be in my old age (if I have no children)?

R. Ami: If this is her concern, we certainly force him.

A woman made a similar claim in front of Rav Nachman. When he heard that she desired a son to support her in her old age and to bury her, he replied like R. Ami.

R. Chiya's wife had a very painful pregnancy. She disguised herself and came before R. Chiya. She asked whether or not a woman is commanded Peru u'Rvu; he said that she is not. She drank a sterility potion.

66a - Question: A woman is commanded to have children!

(Rav Acha bar Rav Ketina): A case occurred in which Rabanan forced the owner of a half-freed Shifchah to free the rest of her (so she could marry and have children)!

Answer: She is not commanded. The half-slave was freed because people were freely having Bi'ah with her.

Rishonim

Rif: If a man married a woman and she did not have children in 10 years and he does not want to marry someone else, he must divorce her and pay a Kesuvah. The Halachah follows Rava who says that he may marry another woman without divorcing the first if he can support two wives.

Rif (Yevamos 20b): R. Ami's law pertains to the Mishnah and Beraisa (Yevamos 64a) that say if one was childless for 10 years he must divorce her and pay a Kesuvah. If he says that she is sterile and she says that Eino Yoreh k'Chetz, he may not divorce her without a Kesuvah.

Question (Ba'al ha'Ma'or): Even if he is Yoreh k'Chetz, he must pay a Kesuvah because he did not merit to have children through her! Since he is commanded about Peru u'Rvu, he was punished! R. Ami does not teach about a Kesuvah, rather, whether or not he must divorce her. We believe her that he is sterile, so he need not divorce her. Alternatively, in a case where he fulfilled Peru u'Rvu through a previous wife and she says that now Eino Yoreh k'Chetz and she needs children for her old age, he must divorce her and pay a Kesuvah.

Answer (Milchamos Hash-m): After 10 years it is more reasonable to say that she is sterile than to say that he is being punished. If he already fulfilled Peru u'Rvu surely she is being punished. Even if she does not say that she needs children for her old age, we know that she does.

Rambam (Hilchos Ishus 15:7): If a man married a woman and she did not have children in 10 years, he must divorce her and pay a Kesuvah or marry a woman who can conceive.

Rambam (14): If a woman did not give birth in 10 years in each of two marriages, she may not marry a third man. If she did, she leaves without a Kesuvah unless he has another wife or he already fulfilled Peru u'Rvu.

Magid Mishneh: If he has another wife or he fulfilled Peru u'Rvu he need not divorce her, therefore if he chooses to divorce her he must pay a Kesuvah.

Rosh (13 and Kesuvos 4:3): If a woman was childless after 10 years with each of two men and she married a third man who did not have children, he divorces her without a Kesuvah. This implies that she receives a Kesuvah from her second husband. The Gemara asks whether or not her first two husbands may demand back the Kesuvah if she is childless also from the third man. This shows that they paid Kesuvos, even though the Tana is Rebbi, who establishes her to be barren after two marriages. This is because we do not rely on Chazakah for monetary matters until three occurrences, like Shor ha'Mu'ad. She does not get a Kesuvah from the third even if he divorced her before 10 years, because it was a Mekach Ta'os. The Gemara said that if she had children from a fourth man and demands a Kesuvah from the third, he could say that he would not have divorced her had he known. If he was with her for 10 years, he would have no choice! If she did not have children from the third husband, she need not return the two Kesuvos she collected, for she says 'now I became weak.' It seems that this is only if she already collected. If not, she cannot make them pay. We do not say that Vadai she was healthy beforehand. If so, she would be allowed to marry a third man!

Hagahos Maimoniyos (15:9): R. Yo'el says that even if her third husband knew her history she has no Kesuvah. In any forbidden marriage that does not disqualify her or her child, she entices him so she has no Kesuvah. Ra'avan disagrees. Since he is commanded about Peru u'Rvu, he transgressed, so he pays a Kesuvah.

Poskim

Shulchan Aruch (EH 154:10): If a man married a woman and she did not have children in 10 years, he must divorce her and pay a Kesuvah or marry a woman who can conceive.

Question (Beis Shmuel 22): This is like most Poskim, who say that he must pay a Kesuvah even if she does not claim that Eino Yoreh k'Chetz, unless he has a definite claim why she is unable to become pregnant. However, in Sa'if 6 the Shulchan Aruch rules like the Rambam, who obligates a Kesuvah only when she makes this claim!

Beis Meir (6 DH Aval): The Tur and Shulchan Aruch hold like the Ba'al ha'Ma'or, but not like the Milchamos understood. R. Ami discusses when he previously fulfilled Peru u'Rvu, not so we will attribute the punishment to her sin, rather, to explain why Beis Din would not force him to divorce without her claim. We attribute the punishment to his sin, since only he is commanded, unless she claims that she needs children for her old age. Therefore, only when she demands a Get for the sake of children she must claim Eino Yoreh k'Chetz to get a Kesuvah.

Shulchan Aruch (17): If a woman did not give birth in 10 years in each of two marriages, she may not marry a third man. If she did, she leaves without a Kesuvah unless he has another wife or he already fulfilled Peru u'Rvu. If he knew her history she receives a Kesuvah.

Beis Shmuel (33): If her third husband has another wife or already fulfilled Peru u'Rvu she receives a Kesuvah even if he did not know her history.