Ok, trying to answer two different threads...
First "viability" has never been an adequate definition of personhood. The definition is entirely arbitrary and effectively strips premies who require intense medical assistance in order to survive of "personhood". They are not non-persons who become persons through successful medical treatment. It also has nasty implications for other related issues, if someone who cannot survive by themselves is a non-person then in all liklihood some of your relatives will lose their personhood, and therefore the implied right to continue to live, prior, to their death.
In any event, this is ultimately semantics, what we as a society are searching for is an excuse for doing what we want, namely to vest the right to decide whether a child lives or dies in the whims of their mother. However, because we still have a few remaining scruples, we also want to reserve the right to pick an arbitrary point at which the mother no longer has a right to take the life of her offspring. But practical experience indicates that pro-abortion groups object to any attempt to implement such a impediment on their ability to legally take a life. This was evident from the fight over partial birth abortions.
Ultimately the issue terminates in your view of the origin of life. If we are just the result of what evolutionary psychologist Stephen Pinker called the "primate assembly process" then any notion of "rights" is illusory from the beginning since there is no possible source for a right. We are simply organisms created by time and chance. What we really have is abilities. We are able to do some things and not able to do others because of constraints either physical or legal. Legal constraints in such a system would be based on preference. Those in power currently prefer the right to kill our inconvenient offspring, and since those offspring are simply organisms not essentially different from any other - brief blips of life in the continuum of death - then why shouldn't we do so? For that matter why shouldn't we end any life that has become inconvenient or too expensive? We have the power to do so, and ultimately power combined with preference are all that matter. We can create pragmatic arguments to make ourselves feel better later on.
If on the other hand, man is not just the result of time and chance but a being created in the image of God, and if He alone is the source not only of Right and Wrong, then we should not end a life on any grounds except for those He has established.
Finally Torque, I don't sense you asked the question seriously, but the biblical teaching on the matter is that we are conceived in a fallen and sinful condition (Psalm 51:5 or Eph. 2:1-7) and therefore the answer is from the very beginning, we have a fallen nature and an inclination towards sin. However, that condition doesn't mean that we have a right to put anyone to death before they have committed a crime worthy of that sentance. Quite the opposite, we have a duty to preserve life, especially at its most vulnerable.
To quote John Calvin on the subject: "the foetus, though enclosed in the womb of its mother, is already a human being, and it is almost a monstrous crime to rob it of the life which it has not yet begun to enjoy. If it seems more horrible to kill a man in his own house than in a field, because a man’s house is his place of most secure refuge, it ought surely to be deemed more atrocious to destroy a foetus in the womb before it has come to light."
- SEAGOON
