אבן העזר י״גEven HaEzer 13

Se’if 1א׳

כָּל אִשָּׁה שֶׁנִּתְגָּרְשָׁה אוֹ שֶׁנִּתְאַלְמְנָה, הֲרֵי זוֹ לֹא תִּנָּשֵׂא וְלֹא תִתְאָרֵס לְאַחֵר, עַד שֶׁתַּמְתִּין תִּשְׁעִים יוֹם חוּץ מִיּוֹם שֶׁנִּתְגָּרְשָׁה אוֹ שֶׁמֵּת בַּעְלָהּ בּוֹ וְחוּץ מִיּוֹם שֶׁנִּתְאָרְסָה בּוֹ, כְּדֵי שֶׁיִּוָּדַע אִם הִיא מְעֻבֶּרֶת אוֹ אֵינָהּ מְעֻבֶּרֶת, לְהַבְחִין בֵּין זַרְעוֹ שֶׁל רִאשׁוֹן לְזַרְעוֹ שֶׁל אַחֲרוֹן. אֲבָל מֻתֶּרֶת לַעֲשׂוֹת שִׁדּוּכִים בְּלֹא אֵרוּסִין (ב"י בְּשֵׁם תְּשׁוּבַת הָרֹא"שׁ וּת"ה סִימָן רי"ז), רַק שֶׁלֹּא יִכְנֹס עִמָּהּ בְּבַיִת (שָׁם). וּמִיּוֹם כְּתִיבַת הַגֵּט מוֹנִים לַמְגֹרֶשֶׁת, וַאֲפִלּוּ הָיָה עַל תְּנַאי אוֹ שֶׁלֹּא הִגִּיעַ לְיָדָהּ אֶלָּא לְאַחַר כַּמָּה שָׁנִים, מִיּוֹם הַכְּתִיבָה מוֹנִים, שֶׁהֲרֵי אֵינוֹ מִתְיַחֵד עִמָּהּ מִשֶּׁכָּתְבוֹ לָהּ. הַגָּה: וְיֵשׁ אוֹמְרִים דְּמוֹנִים מִיּוֹם נְתִינַת הַגֵּט (טוּר בְּשֵׁם הָרֹא"שׁ) וְכֵן רָאוּי לְהַחֲמִיר, כֵּן נִרְאֶה לִי. וּגְזֵרַת חֲכָמִים הִיא שֶׁאֲפִלּוּ הָאִשָּׁה שֶׁאֵינָהּ רְאוּיָה לֵילֵד, וַאֲפִלּוּ נִתְגָּרְשָׁה אוֹ נִתְאַלְמְנָה מִן הָאֵרוּסִין, צְרִיכָה לְהַמְתִּין צ' יוֹם. אֲפִלּוּ קְטַנָּה אוֹ זְקֵנָה אוֹ עֲקָרָה אוֹ אַיְלוֹנִית, וַאֲפִלּוּ בַּעְלָהּ בִּמְדִינַת הַיָּם אוֹ סְרִיס אָדָם אוֹ חוֹלֶה אוֹ חָבוּשׁ בְּבֵית הָאֲסוּרִים, וְהַמַּפֶּלֶת אַחַר מִיתַת בַּעְלָהּ, וַאֲפִלּוּ בְּתוּלָה מֵהָאֵרוּסִין, צְרִיכוֹת לְהַמְתִּין תִּשְׁעִים יוֹם:

Any woman who is divorced or widowed may not get married or engaged to another until she waits 90 days, excluding the day she was divorced or on which her husband died and the day on which she becomes engaged, so that it be known if she is pregnant or not pregnant, to distinguish between the seed of the former and the seed of the latter. Rem"a: But she is permitted to make a commitment for marriage without formal engagement (Beit Yosef in the name of a responsum of the Rosh and Terumat haDeshen 217) as long as he doesn't enter into a house with her. From the day of the writing of the divorce document one counts for a divorcee, even if it was [written] on condition [of going into effect later than the day on which it was written] or she didn't receive it until many years later -- from the day of the writing one counts, for he does not seclude himself with her after writing it for her. Rem"a: And some say that one counts from the day of the giving of the divorce document (Tur in the name of the Rosh) and in accordance with that it is appropriate to be stringent, thus it appears to me. And it is a decree of the Sages that even a woman who is unable to give birth, and even if she is divorced or widowed during the engagement period [when sexual relations with the future husband are forbidden], must wait 90 days. Even a minor or an elderly woman or an infertile woman or a woman who never underwent puberty, or even if her husband is abroad or a eunuch or ill or imprisoned, or [even] someone who miscarries after her husband's death, or even a virgin from the engagement period, [all these] must wait 90 days.

Se’if 2ב׳

יְבָמָה שֶׁמֵּת הַיָּבָם, צְרִיכָה לְהַמְתִּין שְׁלשָׁה חֳדָשִׁים אַחַר מִיתַת הַיָּבָם:

A woman waiting to marry her brother-in-law [after the death of her husband before having children with him] whose brother-in-law dies, must wait three months after the death of the brother-in-law.

Se’if 3ג׳

נִתְעַבְּרָה מֵרְאוּבֵן, וּשְׁנֵיהֶם מוֹדִים שֶׁמִּמֶּנּוּ הִיא מְעֻבֶּרֶת, וְהָלְכָה וְנִתְקַדְּשָׁה לְשִׁמְעוֹן, וְגֵרְשָׁהּ, וְרוֹצָה לְהִנָּשֵׂא לִרְאוּבֵן, צְרִיכָה לְהַמְתִּין אַחַר גֵּרוּשֵׁי שִׁמְעוֹן תִּשְׁעִים יוֹם:

If she was impregnated by Reuven, and both of them agree that from him she became pregnant, and she goes and gets engaged to Shimon, and he divorces her, and she wishes to marry Reuven, she must wait 90 days after divorcing Shimon.