Commentary for Yevamot 4:1
הַחוֹלֵץ לִיבִמְתּוֹ, וְנִמְצֵאת מְעֻבֶּרֶת וְיָלָדָה, בִּזְמַן שֶׁהַוָּלָד שֶׁל קְיָמָא, הוּא מֻתָּר בִּקְרוֹבוֹתֶיהָ, וְהִיא מֻתֶּרֶת בִּקְרוֹבָיו, וְלֹא פְסָלָהּ מִן הַכְּהֻנָּה. אֵין הַוָּלָד שֶׁל קְיָמָא, הוּא אָסוּר בִּקְרוֹבוֹתֶיהָ, וְהִיא אֲסוּרָה בִקְרוֹבָיו, וּפְסָלָהּ מִן הַכְּהֻנָּה:
If one gave chalitzah to his yevamah, and she were found to be pregnant, and she gave birth — if the child lived, he may marry her relatives and she may marry his relatives, [this not being chalitzah, chalitzah having been unnecessary. "Her relatives" and "his relatives" of our Mishnah are those forbidden to a man because of his wife, and those forbidden to a woman because of her husband.], and she is not rendered unfit to the priesthood. If the child did not live [i.e., if it were a miscarriage], he is forbidden to her relatives and she is forbidden to his relatives and she is rendered unfit to the priesthood.
Bartenura on Mishnah Yevamot
English Explanation of Mishnah Yevamot
Bartenura on Mishnah Yevamot
English Explanation of Mishnah Yevamot
If the child is viable, he is permitted to marry her relatives and she is permitted to marry his relatives, and he does not disqualify her from marrying a priest. If the child is viable, meaning there was no miscarriage, then the halitzah was unnecessary, because only a husband who has no children cause his wives to be subject to yibbum. Since the halitzah was not necessary, all of the results of a man performing halitzah for a woman do not occur. He is permitted to marry her relatives and she is permitted to his relatives [mishnah 7 below will list which relatives are forbidden had the halitzah been necessary]. Furthermore, since the halitzah was unnecessary, she is not disqualified from subsequently marrying a priest, as a regular halutzah (a woman who has had halitzah is).