Ohalos 7:5

If the first twin was stillborn and the second was born alive, the live child is ritually clean. If the first was born alive and the second was stillborn, the live child is ritually unclean. Rabbi Meir says that if the twins were in one sac, the live child is ritually unclean, while if they were in two sacs, he is ritually clean.

Ohalos 7:6

If a woman was having difficulty in labor, the fetus may be cut up in her womb and removed limb by limb; this is because the mother’s life takes priority over the life of a fetus. Once the greater part of the child has emerged, however, it may not be harmed because no person’s life take precedence over any other’s.