Well, for the sake of argument, I put the line there because once birth happens, pregnancy has ended and can't be aborted.
I don't know where my tolerence would be post-birth. I don't know enough information to justify one thing over another. If I had to say something off the top of my head though, I'd say that I'd have no tolerence post-birth. Each baby has its own distinct personality I'm told. That could be justification for existence.
My reasoning for the issue as a whole is that abortion is part of human nature. It is definatly not a recent phenomenon. Early human societies practiced infanticide as a method of self-preservation. In rough times, another mouth to feed was perhaps too much of a burden and thus lives were taken. Abortion is no different. It's merely a more prempative and efficient method of the above. You could argue that our society today is perhaps more "civilized", but I disagree. In times of trouble us humans freak out just like any other animal. Civilization is but a very thin surface. Point being, while "because we're animals" isn't quite a good argument for abortion, "because we're civilized" isn't either.
Burdens nowadays aren't matters of life or death. But I think that the social burden is the same as life is more complicated.