Any "cut-off" point for abortion is always going to be arbitary though. Clearly we're already at the point where it is legally permissible to abort a foetus which could potentially survive outside the womb.
Personally I have much sympathy with the Warnock report, which looked into the use of embryos back in the 1980s. They came to the conclusion that it would be unethical to experiment with embryos of 14 days or more, for reasons including the fact that they could not prove that after that point that the foetus may not feel pain (can't remember what it is called, but something like the neural cord or something starts to form at that stage). Of course this isn't particularly helpful in the pro-choice world, as a woman would only just know that she is pregnant at the 14 day stage.
But then in reality if we are terminating a pregnancy it is never in the foetus's best interests.
I had 2 weekly scans with my first pg. Up to 8 weeks pg the foetus didn't look real. I was pg with a tadpole at 10 weeks, but there was a baby at 12 weeks.