Qitzur Shulchan Arukh – 184:11
A pregnant woman who is difficulty giving birth, as long as the fetus is in her insides, one may divide it [to ease delivery and save the mother] whether by drugs or manually. For as lone as it did not emerge into the air of the world, it doesn’t have a title of nefesh [living soul]. Therefore in order to save the mother we permit cutting it, because it is like one who pursues his friend to kill him. However, once its head emerges, we do not touch it, for we do not push aside a nefesh for [another] nefesh, and this is the nature of the world [for a mother to be put at risk giving birth].