[To continue a grand blogospheric tradition, the following is a guest post from my evil twin Ricardo. Be warned that he has some terribly misguided views. Hopefully commenters will point out the flaws in his arguments!]
All humans are created equal. From the highest genius to the severely mentally disabled, all are equally members of the human kind. It is membership in this kind, rather than one's peculiar individual characteristics, that bestow moral worth. It doesn't matter how smart you are, if you're a human being, then your life matters. Your life matters because you're a human being, not because of your contingent cognitive abilities.
There's no good reason to refrain from extending this to unborn human beings. An individual human life begins at conception. That's a natural fact. All human beings have moral worth in virtue of their kind. That's a moral fact. Put the two together and you reach the pro-life conclusion that fetuses have moral worth, just like you and me. They may yet lack our cognitive capabilities. But we've seen that such superficial individual differences are morally irrelevant. What matters is our underlying commonality, or shared human essence. Undeveloped human beings deserve the same respect as anyone else.
My humanism is not arbitrary. We should extend our moral concern to any relevantly similar kinds of creatures (e.g. elves and hobbits, were such to exist). Further, if humans were relevantly different, say if we had evolved to be identical to chickens, then we would not have had this special moral status. So what is the relevant difference, you ask? Here we find the grain of truth in pro-choice thought: cognitive capabilities really are morally relevant, in some sense. Indeed, that much is undeniable. But we shouldn't jump to the hasty conclusion that the particular cognitive capabilities of individuals are what matters here. Rather, what matters are the general cognitive capabilities of the species-kind.
This teleological conception of kinds is nicely explained by Rad Geek in his guest post here. Note that even if you chopped a leg off every cat in the world, that wouldn't change the fact that cats are four-legged creatures. It would just mean that each actual cat was missing a leg. Superficial maiming cannot affect a cat's underlying nature. Each cat remains a four-legged creature in kind (that's what it is), despite actually only having three legs (that's how it is). Similarly for humans: the rational animal. Some individuals may fail to realize their rational capacity, but this accidental character has no bearing on the more fundamental question of what kind of being they are. They remain human beings, hence rational in kind, and thereby worthy of moral respect.
The argument is especially strong in case of merely temporary failures to exercise cognition. We would not consider it permissible to kill an unconscious person, after all. This is because we recognize the unity of their life: they remain one and the same person, whether sleeping or awake. It doesn't matter what state they happen to be in at the moment you kill them. If you kill the sleeping person, you kill the whole person. Similarly, there's a sense in which fetuses are cognitively advanced creatures, or "persons". Of course, their current time-slice is comparatively underdeveloped. But this very organism, one and the same individual, would naturally develop into a fully mentalistic person. The fetus and the person are one and the same individual, so if you kill the fetus, then (by the law of identity) you kill the person.
This argument is easily misunderstood, especially by those pro-choice advocates with a fondness for straw, so let me clarify. I am not arguing that fetuses have moral worth just because they could ("potentially") be turned into persons. Rather, they have worth in virtue of what they already are. If cloning technology advanced to the point where a person could be made from the DNA in fingernail clippings, this would not elevate the moral status of such clippings. Despite containing human DNA, fingernail clippings are certainly not individual human lives. They could only attain moral worth after being transformed into a different kind of thing (a human being). A fetus is a human being already; no transformation is required, it may simply develop. (Note the distinction: development is change within a kind, in contrast to transformation across kinds. I might develop your argument, or I might transform it into a parody. In the latter case, I've changed it into a different kind of thing.)
In summary: we all typically recognize that moral status accrues to individuals in virtue of their kind, or what they (fundamentally) are, rather than their individual characteristics, or how they (superficially) happen to be. This is reflected in our respect for the mentally disabled, and in the truism that "all humans are created equal". It is a mystery why pro-choice advocates fail to extend this standard understanding to the unborn. Despite the special name, "fetuses" are not really a fundamentally different kind of thing from you or me. They are human beings, simply at an earlier stage of development. It's true that they lack our cognitive characteristics while in this stage of development. But that doesn't detract from their essential nature, their humanity, their membership in the kind of rational animals; neither should it detract from the moral status that they hold in virtue of the kind of beings that they -- we -- are.