this is a hard question. there will be a group of people that are adamant that they know what they would do, and some of them will be right. most of them will never have to live through something that will make them realise that they could be wrong and so they get to live happily in their ignorance.
I was once adamant that there was nothing that would make me consider abortion, and this was after I already had one special needs child. Then I got a high likelihood for downs syndrome. I went for a CVS because that was the logical next step, but they couldn't get a sample size large enough.
the next weeks waiting to be pg enough for an amnio were excrutiating. I was more devastated at the thought that I was capable of having an abortion than I was at the thought that something was wrong with my child.
DS we had the tests and they were clear. DD2 (really DD3 if you count that I miscarried late with my second pg) the nuchal transculency was mistimed and so they couldn't give accurate results, we didn't follow up, we figured that it just didn't matter to us after all.
I think that some people really do know themselves, and that others are naive and only think that they do.