There are those who identify themselves as "pro-choice" because they freely acknowledge that there is a lack of complete consensus about when human life begins. Since such a consensus is lacking, they seem to believe that it's purely a matter of personal choice, and that one choice is as good as another.
I freely acknowledge that such a consensus is lacking if one is talking about the nebulous concept of "personhood" (although it is an utterly disingenuous and falsifiable claim if one is talking about biological life of the type which is genetically different from the parents of the fetus), but I disagree with the premise that since there is no perfect consensus, there therefore is no fundamental principle which would enable us to resolve the dilemma of what should and shouldn't be allowed.
Simply put, I believe that when there is a doubt about whether or not one is involved in the deliberate destruction of an innocent human being, one owes life the benefit of the doubt.
I tell a story I like to call "The Boy in the Barrel". One day, a man on his day off was out in his rural back yard, shooting at empty barrels just to kill time. (This was back in the days before video games and other diversions!) After an hour or so, his wife called him in for lunch.
He decided to resume his shooting after lunch, but just as he was raising his high powered rifle in order to take another shot, a man from next door ran into his yard and loudly shouted for him to stop. Puzzled, the man with the gun asked why he should do so. His neighbor said, "I know that you think that what you're doing is just innocent fun. But what you don't know is that while you were eating lunch, a neighbor boy wandered into your yard and decided for some inexplicable reason to climb into that barrel you're aiming at. If you shoot now, you will most likely seriously injure or kill that boy."
The man with the gun was conflicted. He'd seen no boy in his yard, and he remembered a time long ago when his neighbor had even lied to him. Nevertheless, it was a chance he dared not take. If there was even the slightest chance that he might be killing an innocent human being in the process of resuming his barrel shooting, he would regret that decision for the rest of his days. So he lowered his gun, walked over to the barrel and looked inside. Sure enough, there was the boy, just as his neighbor had claimed. He breathed a sigh of relief, thankful that he'd listened to his neighbor.
My point? Due to the gravity attached to the act of taking a human life, the burden of proof belongs to those who would argue that a particular action will most definitely NOT take an innocent human life. It does not belong to those who would disagree. If pro-choicers can't even agree about where to draw the line about when human life or "personhood" first begins, whereas most pro-lifers believe that the moment of conception is where one should draw the line, then the logical place to draw the line (for all people with consciences) is the moment of conception.
The above line of argumentation, by the way, does not rely on religious doctrine in any way, shape or form for its legitimacy.