Shulchan Arukh Yoreh De'ah · Siman 14

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Seif 1 An animal (that we are slaughtering) that happens to be in the process of giving birth, (and) if the head (of the fetus) protrudes then returns, it is like it is born (and as if it exited its mother) and is not kosher to eat by virtue of the mother's slaughter.Therefore, if it in 8th month of pregnancy whether alive or dead, or in 9th month it is forbidden; if it is in 9th month and alive, it is kosher by its own slaughter.

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He extended his hand and retracted it (meaning before the slaughtering): what emerged from the limb outside is forbidden. But what remained inside and the place of the cut is permitted. If he did not retract it before the slaughtering, even the place of the cut is forbidden, but what is inside is permitted, even if it is a minority. Gloss: All this refers only to the matter of it being permitted by the slaughtering of its mother like any other fetus found within it. However, if the animal later gave birth, even the limb that emerged and was retracted is permitted, because its own slaughtering permits everything. This is the conclusion of the Beit Yosef and Rabbi Yosef Ibn Habib, and Kol Bo in the name of some authorities, and this is also proven by the Maharshal in Perek Behemah Hamakshah, Siman 1, and this is the implication of the poskim, and it is obvious.

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Seif 3 If the majority of the limb comes out, it is as if it was born. And if half of the whole limb, from the majority from the small limb, it is like it is born.

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Seif 4 If one limb that comes out and went back in and then another limb extends out completely and then majority of it goes back in, it is allowed; and if the whole limb is cut and the limb in an hour returns itself, also a small portion remains inside. (it is allowed)

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Seif 5 Whenever an animal thrust its limb and then returned it and that limb becomes forbidden, and afterwards its mother was slaughtered, If it is female, we are forbidden to drink its milk because the milk comes from all of the animal's limbs and it has a limb which is forbidden. Hence, it is comparable to milk from a trefe animal that becomes mixed with milk from a kosher animal.

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Seif 6 If a person inserts his hand inside of an animal, cuts off the spleen, and or one of the kidneys or the like, but leaves (the cut organ) inside the animal, and then slaughters it, the pieces cut off are forbidden as Ever min HaChai although they remained within the animal's womb. Although if he cut away a section of the fetus within the womb, but did not remove it, and then slaughtered (its mother), the pieces or limbs of the fetus are permitted because they did not emerge (out of the mother).