If I were to give my opinion on a cut-off point, it would be after 12 weeks (with the caveat that most abortions should be performed before that time) but prior to 24 weeks, and with exceptions for gross abnormalities. I don't think it's a question that can have an absolutely strict answer unless one's opinion is that it is always wrong.
For example, I have a couple of friends who have been trying for a child for many years, and the woman is now pregnant after IVF. Tests have shown that the foetus has a chromosomal abnormality, and they are almost at 12 weeks and are finding their decision to be an almost impossible one to take and not one which they feel they can rush.