That's where I would draw the line too. Although I'd also consider up to the point of birth on the basis of personal autonomy. But I balk at drawing the line that far along. But is that balking a reasoned position or purely an emotional one? I'm not sure.
Well, what about within seconds of giving birth? Some abortion machine or doctor with a big hammer is ready to smash the newly delivered baby? Is that significantly worse than killing the same baby just minutes earlier while it was still inside the woman? The vagina as a magical gateway...
Viability might become a different issue in the future. What if technology advances to such an extent that viability comes down to a few weeks after conception? We can't make a functional artificial womb yet, but that might become possible at some point in the future.
Yup indeed, it's perhaps a useful red line right now but in future, if you're able to go from say a ball of cells through to a baby ready for the outside world in an artificial womb then it becomes rather moot and what people are really interested in is more how much of a human is this thing at this stage... at the ball of cells stage (embryo -> blastocyst) I'd say that most people don't care to give it any rights, I mean IUDs, morning-after pills deal with that. The heartbeat at 6 weeks thing that some Republicans have latched onto seems dubious too, it's not really a heart at that stage.
It's really going to be some later stage where it has developed more, where it perhaps would be closer to being viable today... personally, I think the cut-off should be a little bit before viability (in today's tech) and should tech improve I don't see any reason to change that.
Like a ball of cells is disposed of quite regularly in women who say have a coil device fitted as a contraceptive... should they be forced instead to have those removed and put in an artificial womb? Obvs I'd say the vast majority of people wouldn't think so. So that a ball of cells might be viable to be grown in an artificial womb in future and therefore technically the point of viability becomes 0 weeks should probably render viability moot as a line in the sand, it's just perhaps a useful one to refer to these days.