Yevamot 41B

מפני האיבול.

due to the mourning period she must observe for her deceased husband.

גמ׳ בשלמא לא תתייבם: שמא יהיה הולד בן קיימא, וקא פגע באיסור אשת אח דאורייתא. אלא לא תחלוץ, אמאי?

Talmud: The Talmud questions the Mishnah’s opening clause: Granted, she should not enter into levirate marriage, as she might already be pregnant, which would still not be noticeable during the first 3 months, and perhaps the baby will be viable. In that case it would emerge that she was never subject to the mitzva of levirate marriage, and therefore, if the yavam consummates a levirate marriage he will have encountered the Torah prohibition against having sex with one’s brother’s wife. However, why should she not perform ḥalitza?

לימא תיהוי תיובתא דרבי יוחנן, דאמר: חליצת מעוברת שמה חליצה. ולאו מי אותביניה לרבי יוחנן חדא זימנא?! לימא מהא נמי תיהוי תיובתא!

The Talmud suggests: Let us say that this Mishnah, which states that a pregnant woman should not perform ḥalitza, is a conclusive refutation of the opinion of R' Yoḥanan, who said that a ḥalitza performed with a pregnant woman who later miscarries is considered a valid ḥalitza. The Talmud asks: But didn’t they already conclusively refute the opinion of R' Yoḥanan one time previously from other sources? The Talmud rephrases the suggestion: Let us say that from this Mishnah as well there is a conclusive refutation of the opinion of R' Yoḥanan.

לא, הכא היינו טעמא: שמא יהא ולד בן קיימא, ונמצאת אתה מצריכה כרוז לכהונה.

The Talmud deflects this suggestion: No, it is possible that in the Mishnah here, this is the reason that she may not perform ḥalitza: Perhaps the offspring will be viable, in which case although she performed the act of ḥalitza, it is entirely meaningless since it was completely unnecessary, and she would remain permitted to marry into the priesthood. However, this could lead to a problem: People might not realize that the ḥalitza she performed was meaningless, and they would think she is a ḥalutza who is prohibited from marrying a priest. It would therefore emerge that if you permit her to perform ḥalitza even though it is possible that she is pregnant, it could be that you will ultimately require a public announcement to be made for her to attest to the fact that the ḥalitza she performed was meaningless and she is in fact still permitted to marry into the priesthood.