TT #699 · Parashas Ki Seitzei · Seventh Aliyah · Passuk 25:06
And it shall be that the firstborn that she bears shall succeed to the name of his dead brother, so that his name shall not be blotted out from Israel.
Torah Temimah points to Yerushalmi Yevamot 4:1.
TT lemma: וְלֹא יִמָּחֶה שְׁמוֹ מִשְּׂרָאֵל.
מִי שֶׁמֵּת וְהִנִּיחַ אִשְׁתּוֹ מְעֻבֶּרֶת – יָכוֹל תְּהֵא זְקוּקָה לְיָבָם? תַּלְמוּד לוֹמַר וְלֹא יִמָּחֶה שְׁמוֹ מִיִּשְׂרָאֵל – אֶת שֶׁשְּׁמוֹ מָחוּי; יָצָא זֶה שֶׁאֵין שְׁמוֹ מָחוּי.
English: One who dies and leaves his wife pregnant — might she be bound to the yavam? Scripture says “so that his name shall not be blotted out from Israel” — referring to one whose name is blotted out; this one (the potential child) is excluded, for his name is not blotted out.
On the daf / in context: A pregnant widow is not subject to yibbum until it is known whether the child is viable; if a viable child is born, the zikah never attaches.
Derash protecting the potential child’s “name.”
A pregnant widow is not subject to yibbum or chalitzah until the status of the fetus is determined; a viable child releases her completely.