One potential embarrassment to the the pro-choice position is the spectre that any successful arguments to the effect that abortion is morally permissible are likely to be too powerful. Something seems to go wrong if an argument shows that abortion is morally permissible, but also shows that infanticide must be morally permissible - something that sounds shocking.
I've written about this problem in a couple of places, and in slightly different contexts, and am thinking about it again, having received an invitation from a new journal to submit an article on the right to life.
Imagine that the possession of a right to life is contingent on personhood. I take it that "personhood" means, roughly speaking, something like the following: the capacities for reason and self-consciousness, including a concept of the past and future (this bundle of capacities seems to go neatly together as one complex). In that case, no embryo or fetus possesses the right to life, since it lacks personhood. However, it appears undeniable that young babies also lack personhood, as defined. Yet most of us are shocked at the idea of killing a young baby (at least in any common circumstance - leave aside issues of severe disability, for example). The situation seems to be intellectually satisfactory.
In resolving the problem, we should ask ourselves why we are shocked by the idea of killing a young baby, and whether we have any reason to wish we were not shocked in this way. It seems to me that the answers to those questions might be surprisingly complex, but surely our response is something that we will consider justified at the end of the day. Note that I am not talking about epistemic justification of an abstract truth claim such as, "It is morally wrong to kill young babies." I am suspicious of abstract truth claims like this, unless they are actually intended as shorthand for something else. Rather, I am talking about instrumental justification of the emotions of shock, or horror, combined with pity and anger, when people so much as contemplate such an action - together with social attitudes of repudiation and willingness to punish. All of this seems well justified to me - justified against widespread and fundamental values that most of us actually do share. The things we value include the natural love of parents for their children, the hopes of communities for their future, and so on.
The same might apply, to a lesser extent, if we consider a very late abortion. However, it's difficult to see how it could apply to early abortions, much less to such things as the use of human embryos in stem cell research. In such cases, the instrumental justifications for horror, shock, pity, and anger are not present in the same way . I try to explain some of this in more detail in my Journal of Medical Ethics article, "Stem cell research on other worlds".
From the point of view of strict intellectual analysis, the situation is satisfactory after all. What is less satisfactory, perhaps, is that resolving the problem uses a method of philosophical analysis that requires people to step (temporarily) out of their ordinary moral thinking. For practical purposes, is this asking too much of them?