Gemara

(Mishnah): If any of these Arayos died, did Mi'un (annulled a mid'Rabanan marriage), was divorced, or was found to be an Ailonis, the Tzaros are permitted;

Mi'un does not apply to the mother or grandmother of the Yavam's wife.

12b (Rav Bivi - Beraisa): A minor has Bi'ah and uses a contraceptive wad, lest she become pregnant, and then she might die.

Inference: This implies that a minor can get pregnant and not die.

Question: If so, it is possible for a mother-in-law to do Mi'un (i.e. she gave birth before 12). The Mishnah says that this cannot be!

Answer #1: The Beraisa should say 'lest she get pregnant and (surely) die';

(Rabah bar Livai): Before this age (11), she will not get pregnant at all. During this time (11 to 12), she and the fetus will die. After (12), she and the fetus will live.

Objection: Rabah bar Shmuel taught that one cannot say that the mother-in-law did Mi'un... because she already gave birth! (He did not say 'because she is already an adult.' This implies that a minor can give birth, just she may not do Mi'un afterwards.)

Answer #2 (Rav Safra): Our text '... and then she might die' is correct. (Bearing) a child is like Simanim (of adulthood);

Some say that a child is better than Simanim.

Question: How is it better?

Answer: Even R. Yehudah, who permits Mi'un (after Simanim) until she has many hairs, agrees that she may not do Mi'un if she has given birth.

Answer #3 (Rav Zvid): She cannot have children without bringing Simanim.

Question: We can check if she brought (why must we rely on a Chazakah?)!

Answer: We are concerned lest they fell out.

Question: This is like the opinion that is concerned lest Simanim fell out. How can we answer for the opinion that is not concerned?

Answer: Here even he is concerned lest they fell out due to the pain of childbirth.

Kesuvos 39a - Question (Rava): Does a girl receive the law of a Bogeres after death, so that the Kenas (she receives if she was raped or enticed) will pass to her child?

Question: Rav Bivi's Beraisa teaches that a minor cannot become pregnant (so she cannot give birth before Bagrus)!

Suggestion: Perhaps she became pregnant and gave birth while she was a Na'arah.

Rejection: Shmuel taught that Na'arus lasts only six months. She cannot give birth in six months!

Kesuvos 53b - Question: If a girl did Mi'un, is she fed?

Answer (Rav Sheshes - Beraisa): A widow, divorcee or Yevamah in her father's house is fed;

R. Yehudah says, she is fed only if she is still in her father's house.

Question: The first Tana said the same!

Answer: Rather, they argue about a Mema'enes. The first Tana says that she is fed; R. Yehudah says that she is not.

Rishonim

Rif and Rosh (1:7): Rav Safra taught that a child is like Simanim; afterwards, she cannot do Mi'un. Some say that a child is better than Simanim, i.e. even R. Yehudah does not allow Mi'un after she gave birth. Rav Zvid taught that she cannot have children without bringing Simanim. We cannot simply check, for everyone is concerned lest Simanim fell out due to the pain of childbirth.

Rosh (ibid.): Rav Zevid does not say that it is impossible to have children without Simanim. If so, the Gemara would not have asked why we don't check! Also, 'we are concerned lest Simanim fell out' connotes that we are not sure. Rather, it is not common to have viable children without Simanim, therefore a mother-in-law cannot do Mi'un.

Yam Shel Shlomo (1:8): The Rif agrees with the Rosh that Rav Zevid says that children make her a Safek adult. This is why the Rif cites Rav Safra and Rav Zevid without ruling like either of them.

Rambam (Hilchos Ishus 2:9): If a girl gave birth after 12 years, even if she did not bring any Simanim, she is an adult. Children are like Simanim.

Magid Mishneh: Perhaps the Rambam holds that it is impossible to give birth before 12. Alternatively, the one who said that children are like Simanim even before 12 holds that (Simanim) before the Perek (12, the usual time Simanim come) is like after the Perek. This is not the Halachah. Rather, it is possible to have children before 12, but it does not make her an adult. However, if so a mother-in-law can do Mi'un! Rather, we must say that she cannot do Mi'un after birth, but she can during pregnancy. Others say that children are Simanim of adulthood from the beginning of pregnancy. Kesuvos 100b supports them (it seems that one who does Mi'un need not wait three months before remarrying, i.e. because we are not concerned lest she is pregnant).

Nimukei Yosef (Yevamos 2b DH Afilu): Tosfos says that Rav Safra holds that children are a Siman of adulthood from the beginning of pregnancy. This explains why the Gemara (Kesuvos 39a) says that a girl cannot give birth before Bagrus. However, perhaps this is like the version that children are better than Simanim, but the version that considers children to be like Simanim holds that they are a Siman only from birth.

Rambam (19:14): Bas ha'Mema'enes (the daughter of a girl who did Mi'un) is like other daughters. She is fed (from her father).

Rebuttal (Ra'avad and Ran Kesuvos 20a DH d'Tana): A Mema'enes cannot have a child. This is why a mother-in-law cannot do Mi'un (Yevamos 12b)!

Defense #1 (Yam Shel Shlomo ibid.): The Rambam rules like Rav Zevid. He holds that children after 12 make her a Vadai adult, but children before 12 make her a Safek adult. By doing Mi'un she shows that she considers herself a minor and pardons her Kesuvah, but her daughter does not lose from her mother's admission.

Defense #2 (R. Chaim ha'Levi on the Rambam, 2:9): The Rambam agrees that children are a Siman of adulthood from the beginning of pregnancy. However, this is only if she and the child live. The Siman is seen only at birth, so she is not an adult retroactively. However, birth proves that she had Simanim more than six months ago, so she becomes a Bogeres immediately. Even though the Simanim did not make her an adult at the time, we count the six months from then.

Poskim

Shulchan Aruch (155:12): A girl can do Mi'un until bringing two hairs after 12 years, or until she gives birth.

Beis Yosef (DH u'Mah she'Chosav O Ad): Even those who say that sometimes a minor can give birth, since she is a Safek adult she cannot do Mi'un.

Beis Shmuel (18): It seems that even if there is no Safek mid'Oraisa, i.e. she did not have Bi'ah after 12 years, she cannot do Mi'un. The Rif and Rosh bring Rav Zevid. This shows that they hold like him. Even if she gave birth after 12, if she idolatry bring hairs she is a Safek adult.

Rema: From the time of pregnancy she is an adult.

Beis Shmuel (18): Even Tosfos (who says that she is an adult from the beginning of pregnancy) agrees that if she miscarried she may do Mi'un.

Shulchan Aruch: Before bringing two hairs or giving birth she is a minor.