Gemara

Mishnah 82a: If a pregnant woman smelled food (on Yom Kipur and she and the fetus are in danger due to her craving), we feed her until she recovers.

Beraisa: If a pregnant woman smelled pork or Kodesh meat (and is in danger), we put some gravy on the end of a stick and let her suck it. If she recovers, fine. If not, we feed her some of the gravy. If this does not suffice, we feed her the meat itself, for the only things that override Piku'ach Nefesh are Avodah Zarah, Arayos and murder.

82b: People told Rebbi about a pregnant woman who smelled food. He told them to whisper in her ear that today is Yom Kipur; she recovered. He applied to the fetus "B'Terem Etzarcha va'Beten". The baby born was R. Yochanan.

People told R. Chanina about a pregnant woman who smelled food. They whispered in her ear; it does not help. He applied to the fetus "Zoru Resha'im mi'Rechem". The baby born was Shavtai the produce hoarder (he did so to jack up prices).

Erchin 7a - Rav Nachman: If a pregnant woman sat on the birthing stool and died on Shabbos, we bring a knife, cut her open and remove the fetus (perhaps it will live).

Question: This is obvious! (Even if the fetus is dead, we did not transgress, for) we merely cut dead flesh!

Answer (Rabah): He permits to bring a knife through a Reshus ha'Rabim.

Question: Presumably, he teaches that we may Mechalel Shabbos for Safek Piku'ach Nefesh. We already learn this from a Mishnah!

Mishnah: If a house caved in, we unearth it (even if we must Mechalel Shabbos), even if we have the following doubts:

Perhaps no one was inside, and (even if someone was inside) perhaps he is already dead, and perhaps he is a Nochri.

Answer: One might have thought that this is only when there is a Chazakah (we know that a live Yisrael was there. Here, the fetus has no Chazakah, for perhaps it is a Nefel.)

Kerisus 13a: Chachamim permit a pregnant woman (who smelled forbidden food and is in danger) to eat a partial Shi'ur on account of danger.

Question: If there is danger, she should be allowed to eat more!

Answer (Rav Papa): It means, they permitted a pregnant woman to eat a partial Shi'ur or even more on account of danger.

Rishonim

The Rif and Rosh (8:13) cite our Mishnah and Gemara.

Rambam (Hilchos Shevisas Asor 2:9): If a pregnant woman smelled food on Yom Kipur, we whisper in her ear that today is Yom Kipur. If she recovers, fine. If not, we feed her until she recovers.

R. Mano'ach: We remind the woman of the punishment (for a healthy person) who eats on Yom Kipur, and that on Motza'ei Yom Kipur she may eat as much as she desires. If she eats, once she recovers she must resume fasting. We do not say that since she broke the fast she may eat more.

Rambam (Hilchos Ma'achalos Asuros 14:14): If a pregnant woman smelled a forbidden food such as Kodesh meat or pork, we feed her some of the gravy. If this does not suffice, we feed her less than a Shi'ur of the food itself. If this does not suffice, we feed her until she recovers.

Ramban (cited in Rosh 8:13): We begin with a bit of gravy on a stick only for a pregnant woman, for we do not know what will satisfy her. But when doctors say that a Choleh needs to eat, we immediately give the food they suggest, just we try to give the lightest Isur. In Kerisus we learn that we give a pregnant woman less than a Shi'ur at a time so it will not join to a Shi'ur within Kdei Achilas Pras. The same applies to a Choleh.

Rosh (ibid.): It appears that we give a Choleh as much as the doctors think he needs, just like a pregnant woman may eat more than a Shi'ur in Kdei Achilas Pras if she is not placated with less.

Rosh (ibid.) BaHaG permits a pregnant woman to eat even if the danger is only to the fetus, even if it is possible that it is a Nefel. This is like we learned in Erchin (7a), that we Mechalel Shabbos for a fetus even though we do not know whether or not it is viable. Others permit her to eat only because a miscarriage would be dangerous for her. In Erchin we are Mechalel Shabbos because the baby is considered to be born already.

Question (Rosh ibid.): Why must we investigate whether or not the Heter depends on danger to her? In any case, if she does not eat both of them are in danger!

Answer (Korban Nesan'el 20): It is relevant to a woman who is Muchzekes to miscarry without danger to herself.

Note: Perhaps the Rosh holds that there is no such Chazakah, even if she has miscarried 'easily' many times. There is an opinion that Vestos are mid'Rabanan. I.e., even if a woman becomes Nidah every Rosh Chodesh, mid'Oraisa there is no Chazakah that this will continue.

Poskim

Shulchan Aruch (OC 617:2): If (on Yom Kipur) a pregnant woman smelled food (Rema - and her face changed color, even if she does not say that she needs to eat) we whisper in her ear that today is Yom Kipur.

Beis Yosef (DH Kosav R. Yerucham): The Ramban says that if we see that she paled due to smelling a food we feed her even if she does not request it. The Tur writes that this applies to anyone, not only a Me'uberes.

Bach (DH u'Mah she'Chosav v'Chen): The Rambam and Rosh hold that if a Me'uberes smelled food and says that she needs it we feed her even if she did not pale, but we feed other people only if they paled. The Ramban forbids even a Me'uberes if she did not pale.

Magen Avraham (1): No, all agree that a Me'uberes is permitted whether or not she paled, and others are permitted only if they paled.

Mishnah Berurah (1): We do not distinguish whether she is at the beginning or end of pregnancy.

Sha'ar ha'Tziyon (1): Rashi says that the fetus craves the food. Perhaps this is only after 40 days.

Shulchan Aruch (ibid.): If she recovers, fine. If not, we feed her until she recovers.

Question: The Gemara says that first we put some gravy on a stick...if necessary we feed her some of the gravy...if even this does not suffice, we feed her the meat. Why does the Tur omit mentioning the stick and the Rambam omits all mention of the gravy?

Answer #1 (Beis Yosef DH u'Mah she'Chosav Rabeinu): We first attempt to satisfy her with gravy when the food itself is forbidden, but not regarding Yom Kipur, for then all food is forbidden. The Tur mentions feeding gravy when the food itself is forbidden, like he continues '...if not, we feed her the Isur itself'.

Answer #2 (Beis Yosef ibid.): Also the Tur refers to a permitted food. He calls it Isur because it is forbidden on Yom Kipur. He says that we first give her gravy, i.e. we see if a small amount will satisfy her. Perhaps the Rambam agrees. He allows to 'feed her until she recovers' if a small amount did not satisfy her.

Magen Avraham (2): The Shulchan Aruch holds like his second answer in the Beis Yosef, that also regarding Yom Kipur we first try gravy on a stick...