Siman 617
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א עֻבָּרוֹת וּמֵינִיקוֹת מִתְעַנּוֹת וּמַשְׁלִימוֹת בְּיוֹם הַכִּפּוּרִים.
1 Laws of one who is Pregnant, Nursing, or Recently Birthed in regard to Yom Kippur and in it is 4 paragraphs. One who is pregnant or nursing fast and complete on Yom Kippur.
ב עֻבָּרָה שֶׁהֵרִיחָה וּפָנֶיהָ מִשְׁתַּנִּים אַף עַל פִּי שֶׁלֹּא אָמְרָה צְרִיכָה אֲנִי (רי"ו נכ"ז); לוֹחֲשִׁין לָהּ בְּאָזְנָהּ שֶׁיּוֹם הַכִּפּוּרִים הוּא; אִם נִתְקָרְרָה דַּעְתָּהּ בְּזִכָּרוֹן זֶה, מוּטָב; וְאִם לָאו, מַאֲכִילִין אוֹתָהּ עַד שֶׁתִּתְיַשֵּׁב דַּעְתָּהּ.
2 A pregnant woman who smells and her face changes even if she doesn't say "I need it (the food she smells"), we whisper to her in her ear that it is Yom Kippur. If she calms down with this remainder, good. If not, we feed her until she calms down.
ג כָּל אָדָם שֶׁהֵרִיחַ מַאֲכָל וְנִשְׁתַּנּוּ פָּנָיו, מְסֻכָּן הוּא אִם לֹא יִתְּנוּ לוֹ מִמֶּנּוּ, וּמַאֲכִילִין אוֹתוֹ מִמֶּנּוּ.
3 All people that smell food and their face changes are endangered if they are not given it. So we give them to eat.
ד יוֹלֶדֶת, תּוֹךְ שְׁלֹשָׁה יָמִים לֹא תִּתְעַנֶּה כְּלָל; מִשְּׁלֹשָׁה עַד שִׁבְעָה, אִם אָמְרָה: צְרִיכָה אֲנִי, מַאֲכִילִין אוֹתָהּ; מִכָּאן וָאֵילָךְ, הֲרֵי הִיא כְּכָל אָדָם. וְיָמִים אֵלּוּ אֵין מוֹנִין אוֹתָם מֵעֵת לְעֵת; כְּגוֹן אִם יָלְדָה בְּשִׁבְעָה בְּתִשְׁרֵי בָּעֶרֶב, אֵין מַאֲכִילִין אוֹתָהּ בְּיוֹם הַכִּפּוּרִים אִם לֹא אָמְרָה: צְרִיכָה אֲנִי, אַף עַל פִּי שֶׁלֹּא שָׁלְמוּ לָהּ שְׁלֹשָׁה יָמִים עַד יוֹם כִּפּוּר בָּעֶרֶב, מִשּׁוּם דִּכֵיוָן שֶׁנִּכְנַס יוֹם רְבִיעִי לְלֵידָתָהּ מִקְרֵי לְאַחַר שְׁלֹשָׁה.
4 One who has recently given birth within 3 days does not fast at all. From 3 to 7 days, if she says she needs to eat, we give her to eat. From after that, she is like any other person. These days are not counted from moment to moment. For example, if she gave birth on the 7th of Tishrei in the evening, we do not feed her on Yom Kippur unless she says she needs to despite that it has not been 3 complete days until Yom Kippur in the evening. Because that since it has entered the 4th day from her giving birth, it is called after 3 days.