Many abortion opponents of the Christian Right variety (and here I'm riffing off of this guest post on Ezra Klein's blog) take the position that an embryo or fetus is a potential human life and therefore has the same moral valence as a fully formed human being. My problem with this approach to the argument is that, taken in a perfectly logical direction, it has approximately the opposite of the result they're aiming for. Supposing every spermatazoa and ovum is a potential human life -- in a world where cloning works, which will arrive in a few years, so is every other human cell, too -- we note that only the real wackos complain when men and women fail to have sex and impregnate those otherwise lost ova (and there's no saving all those millions of extra sperm). As has been pointed out elsewhere, a large fraction of fertilized blastocysts spontaneously fail to implant, and are also lost (this was the subject of a hillarious reducto ad absurdum a little while back, too). We don't hold funerals for these clumps of cells, nor are extraordinary medical procedures invented to save these ultimate preemies. Amanda has made essentially this point on a number of occasions as well.
Therefore if we are going to lump together everything that is a "potential human life" in a single moral class, it would seem that the presumption is that members of this category are essentially without value and can be created and destroyed at will. And now we draw that out. You don't go from potential human to human being at the instant of birth, after all. A newborn baby has the intellectual sophistication and mental life of a brain-damaged lizard, since most of the complex structure of the brain doesn't form for another three to six months, and something recognizable as a human mind doesn't emerge for a few years. Nor, if left to its own devices, will a three-month-old baby survive to grow into a mature human being, either. So to me this "potential human life" argument sounds like a recipe for justifying infanticide through neglect, which pretty much everyone regards as reprehensible.