A very interesting post, Beno. It almost sounds like you've been reading Practical Ethics by Peter Singer. Incase you haven't, I'll sum up his points for you. I'll have to do so very briefly, though, because I have to go out.
Peter Singer is essentially a two-level preference utilitarian, which means he believes the fulfillment of preferences should be maximised, and that one should work out an ethical framework which will guide them in big decisions, but also have a stock of standard answers to everyday dilemmas (based on that framework), so they don't have to give great thought to every decision. If you don't have time to weigh up all the possible consequences of an action, these general rules will serve well.
IE: It's generally wrong to kill, because that thwarts the victims (usually powerful) desire to go on living. On a day-to-day level this results in an almost always valid general rule that one shouldn't kill.
He argues in great length the same point that you do: that conception, the first trimester, and birth are all arbitrary times, and actually have very little bearing on the issue. He suggests - in keeping with preference utilitarianism - that it is only after the child is able to recognise themselves as a being with a future (and thus able to have a desire to keep living) that they can be said to have a "right to life" (to use a term he would loathe). This, apparently, only happens quite some time after birth -- I don't have time to look up the exact figure.
He also has written a long section about why it is illogical to take into account future preferences that may occur but that don't exist yet, which I don't have time to write out, but that you might be able to find online. If you can't, I'll fill you in later.
Basically, he believes abortion, and even post-birth infanticide is justifiable. It's important to note that he doesn't advocate killing babies willy-nilly or anything like that, only that there is no special, intrinsic wrongness in killing an infant. He believes it is the preferences of those closest to the child that should be valued. He gives a lot of examples with regard to certain severely mentally-disabled children, anacephalic children, etc, where the child's life would be short and painful, or life only in a biological sense, etc etc; and where the parents may have particularly good reasons (above just not wanting a baby) to not want a child born, or to want it euthanased.
Hope that makes sense to you. It is a great book-- if you haven't read it, you should. He has some outspoken views, some that superficially seem cruel or callous, but his views are all very well argued, very logical, and (maybe surprisingly) at their core extremely humane and kind.
-Liam