Hullin, Chapter Four, Mishnah Five
Our mishnah continues to deal with the status of a fetus found in its mothers womb when she is slaughtered.
1) If one slaughtered an animal and found in it an eight months fetus, either living or dead, or a dead nine months fetus, he need only tear it open and take out the blood.
a) If he found in it a living nine months fetus it must be slaughtered, and he would thereby [possibly] incur the penalty for it and its young, the words of Rabbi Meir.
2) But the sages say: the slaughtering of its mother renders it permitted.
a) Rabbi Shimon Shezuri says: even if it is eights years old and is plowing the field, the slaughtering of its mother renders it permitted.
3) If he ripped open [the mother] and found in it a living nine months fetus, it must be slaughtered, since its mother has not been slaughtered.
Section one: Rabbi Meir distinguishes between a live fetus found inside its mother after she has been slaughtered and a dead one. A dead fetus is considered to be part of its mothers flesh and therefore when she is slaughtered the dead fetus is permitted. An eight months fetus is considered to be as if it is dead (for it could not live outside of the womb) and therefore it too is permitted when its mother is slaughtered. However, a living nine months fetus is an independent life. When it is found in its mothers womb it too must be slaughtered before it can be eaten. Furthermore, one who slaughters it and its mother on the same day transgresses the biblical prohibition of slaughtering a mother and her offspring on the same day (Leviticus 22:28).
We should note that the implication of this halakhah is that life begins before the fetus leaves the womb. The point at which this is so would seem to be the point at which the fetus would be viable outside of the womb.
Section two: The other rabbis disagree. Any fetus found in its mothers womb is permitted to be eaten without having to be slaughtered itself. All that one would need to do is drain the blood, the same rule which applies to all meat.
Theoretically, as Rabbi Shimon Shezuri points out, one could even eat an eight year old animal without slaughtering it first, if it was born after its mother was slaughtered.
Section three: If he doesnt slaughter the mother, then if the fetus is alive, it will need to be slaughtered. Since its mother was not slaughtered, there is no way to allow this animal to be eaten without proper slaughtering.