Shulchan Arukh, Orach Chayim 617

דין עוברה ומניקה ויולדת ביו"כ. ובו ד סעיפים:עוברות ומניקות מתענות ומשלימו' ביום הכפורים:

“The laws concerning a pregnant woman, and one who nurses a child, and giving birth on Yom Kippur” - Containing four paragraphs.Pregnant and nursing women fast completely on Yom Kippur.

עוברה שהריחה (ופניה משתנים אע"פ שלא אמרה צריכה אני) (רי"ו נכ"ז) לוחשין לה באזנה שיום הכפורים הוא אם נתקררה דעתה בזכרון זה מוטב ואם לאו מאכילין אותה עד שתתיישב דעתה:

A pregnant woman who smells (food and her face changes even though she did not say “I need it”) we whisper in her ear that it is Yom Kippur and if she quiets down when this is mentioned it is fine, but if not, then we feed her (until) she is satisfied.

כל אדם שהריח מאכל ונשתנו פניו מסוכן הוא אם לא יתנו לו ממנו ומאכילין אותו ממנו:

Every person who smells food and his face changes (its color) and he is in danger if they do not give him from it (the food which he smells), we feed him from it (the food).

יולדת תוך שלשה ימים לא תתענה כלל משלשה עד שבעה אם אמרה צריכה אני מאכילין אותה מכאן ואילך הרי היא ככל אדם וימים אלו אין מונין אותם מעת לעת כגון אם ילדה בשבעה בתשרי בערב אין מאכילין אותה ביום הכפורים אם לא אמרה צריכה אני אף על פי שלא שלמו לה שלשה ימים עד יום כפורים בערב משום דכיון שנכנס יום רביעי ללידתה מקרי לאחר שלשה:

A woman who gives birth must not fast at all within three days (of giving birth). From three until seven (days), if she says, “I have to eat,” feed her. From now on (after seven days), behold she is like every other person. And we do not count these as twenty-four hour days, for example if she gives birth in the evening of the seventh of Tishrei, we do not feed her on Yom Kippur (the tenth day of Tishrei) if she does not say, “I need it (food),” even though the three days were not completed for her until the evening of Yom Kippur, since (Yom Kippur) begins the fourth day after her giving birth, this is called after three days.125Yom Kippur in this example is actually the fourth day but the term "day" here is not to be understood as precisely a twenty-four hour day. In other words, according to a strict counting of twenty-four hours, she should be given food on the evening of the tenth of Tishrei which is Yom Kippur, but if the woman does not feel that she needs the food at this time after giving birth she is considered legally already in her fourth day after giving birth where the law states that she may fast unless she feels she must eat. She may not observe the fast up to seven days after giving birth, but after that time has elapsed, she must fast as every other person does. It was felt that she was out of physical danger by this time unless complications arose. (Yom Kippur here is counted as the fourth day after her giving birth.)