פירוש על יבמות 4:1

החולץ ליבמתו. הוא מותר בקרובותיה – for it is not Halitzah, for surely, she does not require Halitzah and the female and male relatives of our Mishnah, that is that those who are prohibited upon the man on account of his wife, and the male relatives who are prohibited to the wife on account of her husband.

Introduction This mishnah discusses the case where a yavam performs halitzah, but then the woman who had halitzah is found to be pregnant with the dead husband’s child.

אם אין הולד של קיימא – that he is premature/non-viable birth.

A yavam performs halitzah for his yevamah, and she is subsequently found to be pregnant and she gives birth:
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).

If the child is not viable, he is forbidden to marry her relatives and she is forbidden to marry his relatives, and he disqualifies her from marrying a priest. If the child was not viable, meaning she had a miscarriage, then the halitzah was necessary. Therefore, all of the results of a valid halitzah do occur. He is forbidden to marry her relatives and she may not marry his. She is considered a halutzah, and therefore she can no longer marry a priest.