There has been discussion somewhere in this tree about viability, but the word itself wasn't used. Viability also has another meaning: the potential to someday be able to live outside the womb. I actually think the latter is more important morally speaking than the former. In a reasonable world, I would think that sensible pro-lifers should agree that if the foetus is doomed one way or another, why prevent an abortion? (Not that pro-lifers in e.g. Texas are sensible.)

But viability as you define it doesn't mean much to me. Consider the earliest point at which the foetus is viable (could potentially survive outside the womb), versus the day before that. On the day before, the parent has the option to wait one day, at which point the foetus will become viable. Now compare this with a different situation: for the price of $20, a certain drug can be used to save a foetus' life. Would you agree that in the latter situation, the foetus is already "viable"; it just needs a little help? If you agree with this, and since waiting 1 day is a similar cost on the behalf of the parent as paying $20, this means, the day before the foetus becomes viable, it's already "viable" -- the word has no meaning.

(If you disagree, and you think that the necessity of $20 drugs before the baby becomes viable means that it's okay to abort it, I find that to be a strange morality, and I'd like to learn more. Or perhaps you think there's something fundamentally different between waiting 1 day and paying $20.)