The embryo has the same moral status as an adult human (the Claim). Medical studies show that more than 60% of all people are killed by spontaneous abortion (a biological fact). Therefore, spontaneous abortion is one of the most serious problems facing humanity, and we must do our utmost to investigate ways of preventing this death — even if this is to the detriment of other pressing issues (the Conclusion).
The Conclusion is clearly false, so we should likewise reject the Claim. (This is effectively just a real-life version of the 'fire in a fertility clinic' case.) Reflection on these cases reminds us that embryonic death is not a significant intrinsic bad, in stark contrast to the deaths of mental persons. So the crudest pro-life positions, which affirm the above quoted 'Claim', cannot survive reflection. I know some abortion opponents are unimpressed by the above considerations though, so I'd like to explore how (or whether) a more sophisticated pro-life position might remain tenable.
In other words: might there be non-value-based reasons to oppose abortion (reasons that do not depend upon the false assumption that embryonic death is a bad thing)? Consider the analogy of lying, or intentionally causing false beliefs, and how the immorality of this is unaffected by the unproblematic ubiquity of 'spontaneous false beliefs'. It is extremely common for people to acquire false beliefs, and while there may be some instrumental reason to remedy this when possible, it's not usually a big deal. Lying, however, may well be -- not because it brings about anything bad or harmful, but because the act itself is disrespectful -- an inappropriate way of interacting with a fellow human being.
I guess one might attempt to make a parallel case against abortion: though embryonic death is harmless in itself, the very act of abortion [typically? occasionally?] expresses an inappropriate disrespect for the sanctity of human life. A virtuous person wouldn't have one. Or something like that. (Personally, I don't see any reason to respect 'human life' as such, rather than the actual people who matter.)
This strikes me as the strongest pro-life position one could reasonably hold. (Any counterarguments?) So it's worth noting just how moderate it is. Hilzoy, in a post exemplifying this view, compares having an abortion to cutting up a corpse. Neither is the sort of thing one should do for frivolous reasons ('just for fun'). But there may well be non-frivolous reasons which could justify such action. Since the moral issue is wholly a matter of respect and symbolism, not actual consequences, it all comes down to the motives of the agent, and how they conceived of their action. As such, it become extremely difficult on this view to insist that all such actions ought to be prohibited as a matter of law.