Solo viability.
You could argue that we don't have solo viability until many months after we're born. Maybe even years. And there are plenty of elderly people who don't have "solo viability".
I'm not a pro-lifer. I'm definitely pro-choice. But both sides of this debate always try to simplify the issue and mark a line in the sand. It's very difficult, if not impossible, to mark that line. Terminating a pregnancy isn't an easy decision. Any attempt to make it easy by saying "it's a clump of cells" is really just trying to avoid the obvious ethical and moral dilemma that abortion represents.
I think that in order to have a meaningful debate, the complexity of the issue has to be acknowledged. Both sides can make valid arguments; neither is completely rock-solid and obviously correct, as far as I'm concerned. I think that as a society we have to acknowledge that abortion is a distasteful thing wrought with emotional and moral implications. That doesn't mean it's wrong. In our society we do lots of distasteful and ethically questionable things, because doing those things allows for the pursuit of a greater good.
--- D