אבן העזר קנ״וEven HaEzer 156

Se’if 4ד׳

מִי שֶׁמֵּת וְהִנִּיחַ אִשְׁתּוֹ מְעֻבֶּרֶת, אִם הִפִּילָה, הֲרֵי זוֹ תִּתְיַבֵּם. וְאִם יָלְדָה וְיָצָא הַוָּלָד חַי לַאֲוִיר הָעוֹלָם, אֲפִלּוּ מֵת בְּשָׁעָה שֶׁנּוֹלַד, הֲרֵי אִמּוֹ פְּטוּרָה מֵהַחֲלִיצָה וּמִן הַיִּבּוּם. אֲבָל מִדִּבְרֵי סוֹפְרִים, עַד שֶׁיִּוָּדַע בְּוַדַּאי שֶׁכָּלוּ לוֹ חֳדָשָׁיו וְנוֹלַד לְתִשְׁעָה חֳדָשִׁים גְּמוּרִים. הַגָּה: יֵשׁ אוֹמְרִים דְּבַזְּמַן הַזֶּה אֲפִלּוּ לֹא נִכְנְסָה בַּחֹדֶשׁ הַתְּשִׁיעִי רַק יוֹם אֶחָד מִלְּבַד יוֹם שֶׁנִּתְעַבְּרָה בּוֹ, הָוֵי וְלַד קְיָמָא. וְאַף עַל גַּב דְּאָמְרִינָן בַּגְמָרָא יוֹלֶדֶת לְתִשְׁעָה אֵינָהּ יוֹלֶדֶת לִמְקֻטָּעִין, כְּבָר תָּמְהוּ עַל זֶה רַבִּים שֶׁהַחוּשׁ מַכְחִישׁ זֶה, אֶלָּא שֶׁאָנוּ צְרִיכִין לוֹמַר שֶׁעַכְשָׁיו נִשְׁתַּנָּה הָעִנְיָן, וְכֵן הוּא בְּכַמָּה דְּבָרִים (ב"י בְּשֵׁם הַרשב"ץ). מִיהוּ, חָדְשֵׁי הָעִבּוּר כָּל אֶחָד שְׁלשִׁים יוֹם, וְלֹא חַשְׁבִינָן לְהוּ כְּסֵדֶר הַשָּׁנִים (כֵּן מַשְׁמָע שָׁם). אֲבָל אִם לֹא נוֹדַע לְכַמָּה נוֹלַד, אִם חָיָה ל' יוֹם, הֲרֵי זֶה וְלַד קְיָמָא וּפוֹטֵר נְשֵׁי אָבִיו מֵהַחֲלִיצָה וּמֵהַיִּבּוּם. וְיֵשׁ אוֹמְרִים דַּוְקָא שֶׁגָּמְרוּ שְׂעָרָיו וְצִפָּרְנָיו (טוּר), וְכֵן נִרְאֶה לִי. וְאִם מֵת בְּתוֹךְ שְׁלשִׁים, אֲפִלּוּ בַּיּוֹם הַשְּׁלשִׁים, בֵּין שֶׁמֵּת מֵחֹלִי בֵּין שֶׁנָּפַל מֵהַגַּג אוֹ אֲכָלוֹ אֲרִי, הֲרֵי זוֹ סְפֵק נֵפֶל סָפֵק בֶּן קְיָמָא, וּצְרִיכָה חֲלִיצָה מִדִּבְרֵי סוֹפְרִים, אֲבָל לֹא תִּתְיַבֵּם. וְיֵשׁ אוֹמְרִים דַּאֲפִלּוּ אִם נוֹלַד מֵת, וַאֲפִלּוּ אִם יֵשׁ לְהִסְתַּפֵּק שֶׁכָּלוּ חֳדָשָׁיו, חוֹלֶצֶת וְלֹא מִתְיַבֶּמֶת (טוּר בְּשֵׁם הָר"פ):

If one died and left behind a pregnant wife: If she has a miscarriage, she becomes someone who needs to do yibbum. If she gives birth, and the fetus emerges alive into the world, even should it die in the hour after it was born, its mother is exempt from chalitza and from yibbum. But on a rabbinic level, [we require] that until it is known for certain that it can live out the month, and that it was born after nine full months. Rem"a: Some say that nowadays, even if it only entered the ninth month of pregnancy by just one day, it is considered a viable fetus. And even though we say in the Talmud that a pregnant woman for nine months cannot be a "pieced" pregnancy [i.e. eight months and a day, for it will die], many have already asked about this how empirical evidence contradicts this. Rather we must say that now, the matter has changed, and similarly for many things (Bet Yosef in the name of the Rshbetz). However, the months of pregnancy are each 30 days, and we don't count them like we count the year [some months 30 days, some months 31, some 29] (so implied ibid.). But if it is not known how long it was born for, if it lives for 30 days, that is considered a viable fetus and the wives of its father are exempt from chalitza and yibbum. Rem"a: And some say only when its hair and fingers grow in (Tur). And so it seems to me. And if it dies within 30 days, even on the 30th day, whether it died from disease, whether it fell off the roof or a lion ate it, that is considered a doubt either a fetal death or a viable fetus, and she needs to do chalitza on a rabbinic level, but not yibbum. Rem"a: And some say even if it was born dead, and even if there is what to doubt it would live out the month, she does chalitza and not yibbum (Tur in the name of the R"P).

Se’if 5ה׳

אִם אֵין יָדוּעַ שֶׁכָּלוּ לוֹ חֳדָשָׁיו אֶלָּא עַל פִּיהָ, אֵינָהּ נֶאֱמֶנֶת. יְבָמָה שֶׁנִּשֵּׂאת לַשּׁוּק, וְעֵדִים מְעִידִים שֶׁהַוָּלָד שֶׁיָּלְדָה אַחַר מוֹת בַּעְלָהּ חָיָה שְׁלשִׁים יוֹם, וְעֵדִים אֲחֵרִים מַכְחִישִׁין וְאוֹמְרִים שֶׁלֹּא חָיָה שְׁלשִׁים יוֹם, מֻתֶּרֶת לְבַעְלָהּ וְאֵינָה צְרִיכָה חֲלִיצָה. וְאִם אֵין עֵדִים מְעִידִים שֶׁחָיָה ל' יוֹם, וְגַם אֵין עֵדוּת בְּרוּרָה שֶׁלֹּא חָיָה ל' יוֹם, אֶלָּא קוֹל לַעַז, לֹא אָסְרִינָן לַהּ בְּקָלָא דְּבָתַר נִשּׂוּאִין:

If they only know that he came to full term by her word, she is not believed. [If] a yibbum woman who went to the marketplace and got married, and witnesses testify that the fetus she gave birth to after her husband died lived for thirty days, and other witnesses contradict them and say that it did not live for thirty days, she is permitted to her [new] husband and she is not required to do chalitza. And if there are no witnesses that testify that it lived for thirty days, and there is no clear testimony that it did not live thirty days, but there is a pernicious rumor, we do not prohibit her based on a rumor after the marriage.