Clearly viability would be where the line should be drawn and as viability comes down with medical advancements so would the line. With exceptions for the health of the mother obviously.
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.
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.