Gemara

(Mishnah): A Yevamah may not do Yibum or Chalitzah within three months of the death of her husband.

Question: Granted, she may not do Yibum within three months, lest she is pregnant with a viable child, and they transgress Eshes Ach mid'Oraisa. Why is Chalitzah forbidden?

Suggestion: Our Mishnah refutes R. Yochanan, who says that Chalitzah of a pregnant woman is valid.

Question: He was already refuted!

Answer: We suggest that also our Mishnah refutes him!

Answer #1: Our Mishnah is no refutation. Chalitzah is forbidden lest she give birth to a viable child, and we will need a proclamation that she is permitted to a Kohen.

Question: What is wrong with that?

Answer: If someone saw the Chalitzah and did not hear the proclamation, he will think that she is forbidden to a Kohen (and perhaps she will marry a Kohen)!

Question: This applies to a regular widow. If she was previously divorced (she is forbidden to a Kohen in any case), she should do Chalitzah!

Answer: Chalitzah deprives her of being fed (from her husband's estate).

Objection: This applies to one who fell to Yibum from Nisu'in. If she fell from Kidushin (she is not fed), how can we answer?

Answer #2: Rather, the reason is like R. Yosi.

(Beraisa): A man asked R. Yosi if he may do Chalitzah within three months. R. Yosi said that he may not.

The man: Why don't we do Chalitzah?

R. Yosi: "If he does not want to do Yibum" implies that he could do Yibum if he wanted. Only one who could do Yibum can do Chalitzah.

Question (Rav Chinena - Beraisa): All doubtful cases do Chalitzah, not Yibum.

Question: What are the doubtful cases?

Answer #1: We do not know whether or not the Kidushin to the deceased was valid.

Rejection: They can do Yibum in any case! (If she was not Mekudeshes, this is regular Nisu'in.)

Answer #2: Rather, a man was Mekadesh one of two sisters, and he does not know which one. The Beraisa says that they do Chalitzah (even though Yibum is forbidden)!

Answer: That is no difficulty! There, if Eliyahu would tell us which he was Mekadesh, she could do Yibum or Chalitzah. Here, even if Eliyahu would tell us that she is not pregnant, we would not heed him and do Yibum!

A minor cannot be pregnant, yet even she may not do Yibum within three months!

(Beraisa): If a Yevamah did Chalitzah within three months, she must wait three months. If she did Chalitzah after three months, she need not wait three months.

The three months are counted from her husband's death, not from the day of Chalitzah.

Question: Why is this different than a Get (according to Rav)?

(Rav): (We count the three months before a divorcee may remarry) from the giving of the Get;

(Shmuel): (We count) from the day it was written.

Answer (Rava): If we permit (Yibum three months after death, even though if she is pregnant they transgress Eshes Ach, which is) Chayavei Kerisos, all the more so we permit (a Chalutzah to remarry three months after death, for Yevamah l'Shuk is only) Chayavei Lavin!

35b (Mishnah): If a Yevamah did Chalitzah and was found to be pregnant, if the baby is a Nefel, he is forbidden to her relatives, and she is forbidden to his relatives and to Kohanim.

Rishonim

Rif and Rosh (4:24): R. Yosi taught that one may not do Chalitzah within three months because only one who could do Yibum can do Chalitzah - "Im Lo Yachpotz..."

Nimukei Yosef (DH Gemara): There are women who cannot do Yibum and do Chalitzah, but they will never be able to do Yibum. Here, after three months she could do either. Shmuel counts the three months from the day the Get was written, for from then they were not secluded together. (If they were, it is a Get Yashan, which is forbidden to give - Gitin 79b.)

Rambam (Hilchos Yibum 1:19): A Yevamah may not do Yibum or Chalitzah until waiting 90 days excluding the day of death and the day of Yibum or Chalitzah, like other women. She cannot do Chalitzah within 90 days because she cannot do Yibum - "Im Lo Yachpotz..." If he did Yibum or Chalitzah within 90 days she is exempt if she is not pregnant. She does not need anything (else).

Rosh (25): If a Yevamah did Chalitzah within three months, she must wait three months. After three months, she need not wait three months (for Havchanah to make it clear who is the father). R. Tam says that this is only when she did Chalitzah, which proves that there was no Bi'ah. This is like Mi'un. A Mema'enes need not wait three months because the Mi'un clearly shows that she is a minor. However, if the Yavam died after three months she must wait three months. A Yevamah is more prone to have Bi'ah than an Arusah, who must wait lest people think that a Nesu'ah need not wait. The three months are counted from her husband's death, not from the day of Chalitzah. After a Get, Rav says that we count the three months from the giving of the Get; Shmuel counts from the day it was written. Here is different. Since we permit Kares, all the more so we permit a Lav.

Question (Tosfos 35b DH v'Nimtzeis): The Mishnah (35b) discusses Chalitzah within three months. If the baby is a Nefel, why is she forbidden to his relatives? Chalitzah within three months is not Chalitzah!

Answer (Tosfos): Mid'Rabanan it is not Chalitzah, but mid'Oraisa it is. Further, b'Di'eved even mid'Rabanan she need not repeat Chalitzah. A Beraisa (41b) says that if she did Chalitzah within three months, she must wait three months. This connotes she need not repeat Chalitzah.

Poskim

Shulchan Aruch (EH 164:1): A Yevamah may not do Yibum or Chalitzah until 90 days from the husband's death, excluding that day and the day of Yibum or Chalitzah.

Beis Shmuel (1): This is even if we know that she is not pregnant.

Shulchan Aruch (ibid.): If he did Yibum or Chalitzah within 90 days she is exempt if she is not pregnant. She does not need anything (else). However, even if she did Chalitzah she may not remarry until 90 days from the husband's death.

Beis Shmuel (2): The Gemara says that if she was found to be pregnant after Bi'ah we separate them, but not if she is Safek pregnant.

Rema: Some say that if she did Chalitzah within 90 days, whether she was found to be pregnant and miscarried or whether she was not pregnant, it is a Pasul Chalitzah and she must do Chalitzah again from all the brothers.

Source (Mordechai Sof Gitin 472): Rabanan disagree about whether or not one who did Chalitzah within 90 days must do Chalitzah again with all the brothers, or if it suffices to repeat Chalitzah with the Choletz after 90 days. We learn from R. Yosi that Chalitzah within 90 days is Pasul. A case occurred with a doctor that shows that Chachamim agree. Also, we hold like Reish Lakish that Chalitzah or Yibum with a pregnant woman is invalid. The same applies to Chalitzah or Yibum within three months. It is not listed amidst Pasul Chalitzos (in the Mishnayos in Perek Mitzvas Chalitzah), because we list only Pesulim in the Chalitzah itself, e.g. with the left foot or at night. The Yerushalmi connotes that it is a Pasul Chalitzah.

Hagahos ha'Tur (1): 'Lechazer Al Kol ha'Achim' (in the Mordechai) means that she needs Chalitzah from one of them.

Beis Shmuel (4): According to the opinion that Chalitzah Pesulah must do Chalitzah with all the brothers, she must do Chalitzah with all except for the one who already did Chalitzah, for his Chalitzah (later) is inferior.

Taz (2): The Mordechai is a lone opinion. We follow the Rambam and Shulchan Aruch. If Chalitzah was done and it was found that she was pregnant and miscarried, she is forbidden to his relatives and to Kehunah (35b). This shows that it is a Chalitzah! A Beraisa (41b) says that if she did Chalitzah within three months, she must wait three months. This is for Havchanah. She need not repeat Chalitzah after three months. Surely the Reisha is parallel to the Seifa, which says that if she did Chalitzah after three months, she need not wait three months, i.e. from the day of death. The only difference is whether or not she must wait, but in either case she need not repeat Chalitzah. The Mordechai says that the Chalitzah is Pasul. L'Chatchilah she should not do Chalitzah, but b'Di'eved it is Chalitzah. This is how R. Yochanan explains the Mishnah; Tosfos (41b DH Kora) says that Reish Lakish agrees. The Mordechai supported himself from the Yerushalmi. Our Gemara surely disagrees. And even the Yerushalmi agrees that if she did not repeat Chalitzah, it was Kosher.

Gra (5): The first answer in Tosfos (35b DH v'Nimtzeis) holds like the Mordechai.