Why is viability used as a standard to decide whether a fetus can be aborted?

There are some practical reasons for using viability as the standard for when a fetus may no longer be aborted. In particular, the viability of a fetus means that its life could be sustained without the mother's involvement -- which makes it much easier for others to effectively intervene. With scientific and technological advances, of course, life outside of the womb has become possible at earlier and earlier stages, so that disallowing abortion in cases where the fetus could survive apart from the mother'sbody would effectively disallow abortion during most of a woman's pregnancy. Moral reasons for emphasizing viability are largely due to the idea that human beings have a right to autonomous existence (an existence which is not dependent on the desires of others) only insofar as they are capable of autonomous existence. (Compare: I have a right to make my own choices only insofar as I am capable of making my own choices.) This rationale is problematic, however, since (a) no...