Life begins at conception - when does personhood?

[Illuminati Primus]

I'm happy with the concept of life beginning at conception (as long as there is no talk of a soul involved) but something that's harder to determine for me is the idea of personhood. Obviously this is a very pertinent question because it has an impact on when you're producing guidelines on abortions, and at what point you seriously attempt to save a child that is born prematurely.

For me a foetus isn't a person until it is has developed to the point where it could feasibly survive outside the mother without heavy medical intervention, so part way through the third trimester. In the first a baby is just a cluster of cells and until it has a brain you're not going to convince me that it has any more of a right to personhood than a tumourous growth. During the second trimester it starts to shape into a baby with a nervous system and limbs but if you took it out of the mother it couldn't survive. It's only towards the end that a baby is fully formed and can stand on its own two feet, so to speak.

Within my view of life and what should attempt to be protected I don't see why a foetus that is at an early stage should be given any special protection; it can't think or feel so aborting it gives me no qualms, and the "it's a person" argument doesn't work with me (saying that I admit I'm a man and don't have to ever face having to do the act). Equally a child that is born so early it must live in highly specialised conditions in an incubator and requires constant supervision is a bit on the boundary. Yes you want to help if you can but I think there are times when nature is telling you to not bother (especially if they're born with conditions that require immediate operations, the kind which probably caused them to be born early and unviable in the first place).

So what does everyone else think?