(Mishnah): If one does Chalitzah to his Yevamah and she was found to be pregnant, and she gave birth:

If the baby is viable (the Chalitzah was meaningless), the Yavam may marry her relatives, she may marry his relatives, and she was not disqualified from Kehunah;

If the baby is a Nefel (not viable), he is forbidden to her relatives, and she is forbidden to his relatives and disqualified from Kehunah.

(Gemara) Inference: [It says that she was found to be pregnant. If she was known to be pregnant,] it is b'Di'eved [to do Chalitzah]. L'Chatchilah it is forbidden.

Support (Mishnah 11): A Yevamah may not do Yibum or Chalitzah until three months [from the death of her husband].

Question: She should be able to do Chalitzah immediately, no matter what the case is!

If [the fetus] is viable, the Chalitzah was meaningless. If it is not viable, she did Chalitzah!

Answer #1 (R. Ze'irah citing R. Chiyah citing R. Bun citing R. Yochanan): It is lest [she give birth to a viable baby, and] she need an announcement that [the Chalitzah was void, and] she is permitted to Kehunah.

[Also] R. Ba citing R. Yakov bar Idi citing R. Hoshayah said, it is lest she need an announcement for Kehunah.

Answer #2 (R. Hoshayah - Beraisa): "Me'en Yevami" - one whom we tell 'do Yibum', we tell him '[if you do not want,] do Chalitzah.' One whom we do not tell 'do Yibum' (e.g. within three months), we do not tell him 'do Chalitzah.'

Question: What is the difference [between these answers]?

Answer: If she was Kesherah, and became a Chalalah, e.g. a widow [had Bi'ah with] a Kohen Gadol, or a divorcee or Chalutzah to a Kohen Hedyot - the one who says, it is lest she need an announcement for Kehunah, since she does not need an announcement for Kehunah (in any case she is Pasul), she may do Chalitzah. (SEFER NIR asks why we must say that she was Kesherah, and became a Chalalah, i.e. through an Aveirah. The same applies if she was divorced);

The one who says, one whom we tell 'do Yibum', we tell him 'do Chalitzah', she does not do Chalitzah.

Question: If he did Chalitzah within three months, does she need [Chalitzah] after three months? (MEICHAL HA'MAYIM - we ask according to the opinion that it is due to announcement for Kehunah. If a verse disqualifies within three months, surely she must do Chalitzah again!)

Answer: We learn from the following [Mishnah]. If a minor did Chalitzah, she repeats Chalitzah after she matures. If she did not do Chalitzah [again], her Chalitzah was Kosher.

(R. Mana, and R. Yitzchak brei d'R. Chiyah citing R. Yochanan): This is like R. Meir;

R. Meir says that we do not do Chalitzah or Yibum with a minor girl, lest she be found to be an Ailonis [to whom Chalitzah and Yibum does not apply].

Just like you say there that even though she did Chalitzah [and it was Kosher, if we see now that she is not an Ailonis], she does Chalitzah [again], also here, even though she did Chalitzah [and it was Kosher, if we see now that she is not pregnant], she does Chalitzah [again]!

Question (R. Yudan): If one did Chalitzah to a pregnant woman and she miscarried, [what is the law]?

Answer: We learn from the following [Beraisa];

Suggestion: If one died and left his wife pregnant, perhaps she is Zekukah to Yibum!

Rejection: "V'Lo Yimcheh Shemo mi'Yisrael" excludes this [man], whose name is not erased (he will have seed).

Suggestion: Perhaps she may remarry [immediately]!

Rejection: "Lehakim l'Achiv Shem b'Yisrael" - [she may not marry] until you know whether [the fetus] is Ben Kayama (viable) or not Ben Kayama (a Nefel).

Suggestion: Perhaps even the wife of a Seris (who is sterile) needs [Chalitzah or] Yibum!

Rejection: "V'Lo Yimcheh Shemo mi'Yisrael" - one whose name would not be erased [had he lived]. This excludes this [man], whose name is erased.

Suggestion: Perhaps [a pregnant woman's] Tzarah may remarry [immediately]!

Rejection: "Lehakim l'Achiv Shem b'Yisrael" - [she may not marry] until you know whether [the fetus] is Ben Kayama or not Ben Kayama;

From here they said that if one married (did Yibum with) his Yevamah, and she was found to be pregnant, her Tzarah may not marry until you know that the fetus is viable, for [if it is viable, the Bi'ah does not exempt, and] a child does not exempt until he goes out [from the womb] into the world;

This is like you say, that [if she was pregnant], her Tzarah may not marry until you know how she is permitted - through pregnancy [with a viable fetus], or through Bi'ah.

The same applies to Chalitzah. Chalitzah exempts, and Bi'ah exempts. Just like you say about Bi'ah (her Tzarah may not marry until you know whether or not the fetus is viable), so you say about Chalitzah.