The subject of life is fairly settled at conception, and it most certainly deserves protection.
The subject of personhood is a bit trickier (from a secular standpoint), but I think it can at least be narrowed down. Viability is often cited as a potential criteria, but that is untenable. Viability varies widely between individual children and can be effected by genetics, by technology, by economic status (if you can afford/have access to better hospitals/health care.) So should personhood be determined in part by these things? Are poor people or the genetically afflicted somehow less than other people? I can't imagine anyone would say so.
Could it be the formation of certain internal structures? The heart? The brain? Humans are sometimes born with half a heart, some with half a brain, some have parts of their brains or hearts removed or their heart replaced in whole or in part with an artificial one. Heart/lung machines can keep someone without a working heart alive, or their heartbeats maintained by technology because their own hearts lack that function, and some with minimal brain function live their lives from beginning to end and no one would ever call any of them non-persons or less than a full person (or they shouldn't, anyway.)
The fact is that what should ultimately be paramount is the protection of life. It is far more important than trauma, it is more important than convenience or the looming dread of responsibility. That's why doctors won't hesitate to amputate a limb to save an individual if they can. If having to endure trauma was more important than life, then it would be better to let it go gangrenous and die so that the patient need not go through that.
But the part that baffles me the most are those who freely admit they aren't sure when they think life/personhood begins (which means to me that they admit they may very well be committing/condoning murder with abortion), but still hold that allowing it is still the safer option. I simply can't understand that kind of reasoning. For those who wish to take a position, the evidence is that life begins at the beginning, and to those sitting on the fence, it is far and above the safer option to take that position as well. If you're right, you have protected millions of lives from a ghastly, murderous fate. You have ensured their chances at a future. If you're somehow wrong, you've inconvenienced and perhaps traumatized millions of new mothers. Some may even die, which is a tragedy, without a doubt. However, while both are unfortunate, there really shouldn't be any question which is the more heinous.