Yevamot 36A

דכל העולה ליבום ⁠— עולה לחליצה, וכל שאין עולה ליבום ⁠— אינו עולה לחליצה.

that anyone who is eligible for levirate marriage is eligible for ḥalitza and anyone who is not eligible for levirate marriage is not eligible for ḥalitza. Therefore, the original understanding of R' Yoḥanan’s opinion, that both the sex and the ḥalitza of a pregnant woman are valid, was accurate.

אלא אמר רבא, הכי קאמר: הכונס יבמתו ונמצאת מעוברת ⁠— הרי זו לא תנשא צרתה, שמא יהא ולד בן קיימא. וביאת מעוברת לא שמה ביאה, וחליצת מעוברת לא שמה חליצה. והולד אינו פוטר עד שיצא לאויר העולם.

Rava therefore provides a different defense of R' Yoḥanan’s opinion: Rather, Rava said that this is what the baraita is saying: In the case of one who consummates the levirate marriage with his yevama under the assumption that there is a mitzva to do so, and then she is found to have been pregnant at the time of the sex, a rival wife of this yevama may not marry lest the offspring be viable, and sex with a woman pregnant with viable offspring is not considered a valid consummation of levirate marriage through sex, and ḥalitza of a woman pregnant with viable offspring is not considered effective ḥalitza. And furthermore, even if the offspring is viable, it does not release her and her rival wives from the levirate bond until it comes into the air of the world, i.e., until it is actually born.

תניא כוותיה דרבא: הכונס יבמתו ונמצאת מעוברת ⁠— הרי זו לא תנשא צרתה, שמא יהא ולד בן קיימא, ואין ביאה וחליצה פוטרת, אלא ולד פוטר. והולד אין פוטרה עד שיצא לאויר העולם.

It is taught in a baraita in accordance with the opinion of Rava: In the case of one who consummates a levirate marriage with his yevama, and then she is found to have been pregnant at the time of the sex, a rival wife of this yevama may not marry lest the offspring be viable. This is because sex or ḥalitza with a woman pregnant with viable offspring does not release a yevama from the levirate bond; rather, the offspring releases her. And furthermore, even if the offspring is viable, it does not release her and her rival wives from the levirate marriage bond until it comes into the air of the world.