Source: This is like we say in Sotah (6a) that a Safek Sotah does Chalitzah, but not Yibum, due to the Kal va'Chomer.

Question: We can resolve this from Sotah 2a! [The Mishnah] says that she does Chalitzah, but not Yibum, and the Gemara says that this is due to the Kal va'Chomer!

Answer: There is [also] another reason, like the Gemara explains there. (The Torah calls the latter husband "Acher", for he brought evil into his home. Surely the Torah does not command to do Yibum to such a woman!)

Question: Granted, she did not do an action to him. However, since Mema'enes is forbidden to him, her Tzarah is like Tzaras Ervah;

How can we compare her to Mema'enes, who is permitted to the brothers?! (Tosfos ha'Rosh - since this is a mere stringency mid'Rabanan, we rely on even flimsy reasoning to permit.)

Explanation: Mi'un uproots the initial marriage.

Question: According to the current Hava Amina, the Seifa is difficult! It says "anyone who could have done Mi'un, but did not do Mi'un, her Tzarah does Chalitzah, but not Yibum."

If it uproots the initial marriage, she should do Mi'un now and her Tzarah will do Yibum! We ask like this below.

Answer: According to the current Hava Amina, we could say that the Mishnah discusses when she is not here now to do Mi'un, or she is an adult [and can no longer do Mi'un];

Below we challenge this because we want to establish the Mishnah even if she is a minor and she is in front of us.

Question: Why didn't we ask from this above, according to the opinion that forbids Tzaras Machazir Gerushaso, and say to him "doesn't this exclude Tzaras Machazir Gerushaso, who is permitted!"?

Answer: He holds that we exclude from "this is" only similar matters;

An Ailonis is like Eshes Shnei Mesim, for both of them were permitted to their husbands;

Machazir Gerushaso was forbidden to her husband.

Question: According to Rava, who permits Tzaras Bito Ailonis even if he recognized that she is an Ailonis [and accepted to be married to her anyway], what is the Chidush to teach in the Tosefta "all of them who were found to be Ailonis, whether in the husband's lifetime or after death"?

Granted, according to Rav Asi, we need to hear about in her husband's lifetime.

One might have thought that since she was found to be Ailonis in his lifetime, perhaps he accepted [to be married to her anyway].

Answer: One might have thought to forbid her when she was found [to be Ailonis] after death, because at the time she fell, she looks like Tzaras Bito;

This is like we forbid above Tzaras Bito Mema'enes [for this reason].

Question: Why is Ailonis different than Mema'enes (regarding their Tzaros)?

Answer #1: There (Ailonis), she lacked Mi'un. Here (Mema'enes), she is not lacking anything.

Answer #2: There is no Kol (publicity) to Mema'enes. At the time of Mi'un, she (her Tzarah) looks like Tzaras Bito;

Ailonis has a Kol. It is known that she (her Tzarah) was not Tzaras Bito.

Question: What was the question against Rava here, from the fact that it says "they were found to be"? We can say that it says "they were found" for a Chidush!

Answer: It is a bigger Chidush when he recognized her [to be an Ailonis]. Therefore, we answer "the Mishnah means that they were."

Explanation #1 (Rashi): They are permitted to use a wad, but other women are forbidden, for this wastes seed, even though they are not commanded Peru u'Rvu.

Support (Nidah 13a - Mishnah): It is praiseworthy for women to check themselves [for blood] often. If a man checks himself often [for emissions], his hand should be cut off!

The Gemara explains that this is because women are not Benos Hargashah (sensation does not cause them to emit their seed).

Inference: If they were Benos Hargashah, it would be forbidden [due to wasting seed]!

Rejection (of Support): We could explain that they are not Benos Isur Hargashah (they have no Isur to cause sensation, for they may waste their seed).

Explanation #2 (R. Tam): Surely, before Bi'ah she may not put a wad, for this is not the way of Bi'ah. When the man ejects his semen onto the wad, it is as if he spills his seed onto wood and rocks;

However, if she puts a wad after Bi'ah, it seems that we do not forbid. The man has Bi'ah normally;

This is like Bi'ah with a minor or Ailonis. There is no Isur to have Bi'ah with them, just because they cannot bear children.

The woman who puts the wad in afterwards is not commanded about wasting seed, since she is not commanded Peru u'Rvu.

Explanation: [Any woman may put a wad after Bi'ah. we must say that the three women listed] have Bi'ah with a wad, i.e. they must use a wad.

Explanation #1 (Rashi): When she becomes pregnant, and [during pregnancy] becomes pregnant [with another fetus], one fetus presses on the other and flattens its form.

Implied question: In Nidah (27a) we say that a pregnant woman cannot become pregnant again [during pregnancy]!

Answer: We must say that she cannot become pregnant with a viable fetus.

Explanation #2 (R. Tam): Sandal is when a woman is pregnant with twins, and she has Bi'ah without a wad, the semen enters between the fetuses and presses them and makes one a Sandal.

Support: Nidah 25b connotes like this, that it is within one pregnancy. It asks, why did we mention a Sandal (regarding Tum'ah of childbirth)? There is never a Sandal without a child!

Citation (25b) Answer: If a female was born with it (the Sandal), indeed [we would not need to mention it]. Here, a boy was born with it.

Citation (cont.) Suggestion: R. Yitzchak bar Ami taught that if the woman was Mazra'as (emits her seed) before the man, the child will be a boy. (If not, the child will be a girl.) Since she gave birth to a boy, also the Sandal must be a boy!

Citation (cont.): The Mishnah teaches that this is not so. We are concerned lest both were Mazri'a at the same time [so the children can be either gender].

Summation of Support: If a Sandal can result from a second pregnancy, why would we think that since one child was a boy, also the Sandal must be a boy?! Also, what forced the Gemara to say that [perhaps] both were Mazri'a at the same time?!

Implied question: Why didn't we say that it taught "perhaps she will die" because it is certain only when she gave birth as a minor, but if she gave birth in adulthood, even if she became pregnant as a minor, sometimes she lives?

A mother-in-law cannot do Mi'un, for if she gave birth as a minor, she surely died. If she gave birth in adulthood, she cannot do Mi'un afterwards! ("Chamoso Mema'enes" connotes that she was already his mother-in-law at the time she did Mi'un, so we cannot say that she did Mi'un before adulthood, and later gave birth - Aruch l'Ner.)

Answer #1: Rav Bivi's Beraisa does not distinguish regarding one who became pregnant in the 12th year, whether she gave birth in adulthood, or while a minor.

Answer #2: It is unreasonable that birth while she is a minor would cause death. Rather, it depends on [the time of] pregnancy.

Answer #3: We answer the truth, like Rabah bar Livai, that whenever she becomes pregnant in this time, she and the fetus die.

Suggestion: "After this time" is after 12 years and one day, even without two hairs.

Rejection (Ri): If so, we find that a mother-in-law can do Mi'un, for one may do Mi'un until the majority of her years (36) if she did not bring two hairs, according to the opinion that is not concerned lest they fell out.

Even according to the opinion that we rule like (Nidah 46a), who is concerned when she had Bi'ah after the time, for it is a Safek mid'Oraisa, we find that a mother-in-law can do Mi'un;

E.g., he had Bi'ah after the time, and specified that it is not l'Shem Kidushin. Then, it is not a Torah Safek.

Explanation: Rather, ["after this time" is after 12 years and a day, and] she brought two hairs. The Beraisa did not bother to specify, for normally there are hairs after this time.

Support: The Yerushalmi says that Rabah bar Livai's reason depends on hairs, and not on years.

Inference: Since it says she'Kevar Yaldu, this implies that she did not bring two hairs.

If she brought two hairs, even without birth [we know that] she is not a minor or Ailonis, due to the hairs!

Inference: This shows that a minor can become pregnant and not die!

Question: What was the Hava Amina of the Makshan? He should question the very Beraisa of Rabah bar Shmuel. Since it is without two hairs, even if she gave birth, she should be allowed to do Mi'un!

Answer (Ri): The Makshan holds that even though we discuss a minor, she may not do Mi'un, since she looks like an adult, and people think that she brought two hairs;

He asks only according to Rabah bar Livai, who says that a minor cannot become pregnant and not die.

Rav Bivi's Beraisa is not difficult. Even though it says "lest she die", which implies that there is a minor who becomes pregnant and does not die, we do not find a mother-in-law who can do Mi'un;

The birth prevents Mi'un, even though she is a minor, like I explained.

Suggested Answer #2: The Makshan was not bothered by the Beraisa itself. He considered that [perhaps] birth is like Simanim (signs of adulthood), therefore she cannot do Mi'un.

He asked only according to Rabah bar Livai, and answered that children are like Simanim from the time of pregnancy, like I will explain. (See the next Tosfos.)

Rejection: If so, he should have explicitly said "pregnancy with children are like Simanim", since initially he considered that birth is like Simanim.

Explanation: [They are like Simanim] from the time of pregnancy;

Rav Bivi's Beraisa properly says "lest she die", for in the 12th year, sometimes she lives, even according to Rabah bar Livai, and then she was an adult from the time of pregnancy.

Implied question: [Even if pregnancy is like bringing hairs,] we hold (Nidah 46a) that "during the time is like before the time", i.e. bringing hairs during the 12th year does not make her an adult!

Answer: Even so, pregnancy with viable children during the time is like Simanim after the time.

Implied question: Rabah bar Livai said "during the time, she dies and the fetus dies"!

Answer: He does not discuss the 12th year, for then, sometimes she lives when she was an adult at the time of pregnancy, like I explained;

Rather, "before the time" is before the year before adulthood. "During the time", is during the year before adulthood. "After the time" is after she matured, like Ri explained at the beginning of the Sugya.

Explanation: Because we want to establish the Mishnah like R. Yehudah, that we cannot find a mother-in-law who can do Mi'un, we say this.

Question: Why do we say that they are better than Simanim? Even if they were only like Simanim from the time of pregnancy, we cannot find a mother-in-law who can do Mi'un according to R. Yehudah!

From the time of pregnancy until birth there is a long time, more than from bringing hairs until an abundance of black (after which R. Yehudah does not allow Mi'un)!

Partial answer: According to the opinion there (Nidah 52a) that "an abundance of black" is until [two] hairs surround [the Ervah] from one end to the other, perhaps this requires more time than from pregnancy until birth;

However, according to the opinion there that it is hairs that lie flat, and it looks like there is more black, this is difficult!

Answer: It seems that if she brought hairs and they fell out, she can do Mi'un, even if she delayed the time for an abundance of black;

The time does not cause [that we do not allow Mi'un]. Rather, we truly require an abundance of black, so she appears like an adult to all through this.

Question: Even without "children are better than Simanim", we can establish the Mishnah like R. Yehudah, for he agrees that if they had Bi'ah after adulthood, she may not do Mi'un, like it says in Nidah (52a)!

This mother-in-law had Bi'ah in adulthood when she became pregnant, for children are like Simanim from the time of pregnancy!

Answer: If not that children were better than Simanim, we would find a mother-in-law who can do Mi'un if she became pregnant [without Bi'ah, through semen] in a bathhouse, or a man had Bi'ah with her explicitly not for the sake of Kidushin.