KJ , I can see why some women's choice to abort or selectively reduce might seem to you, an outsider, ethically questionable or uncomfortable. And I can see why that might make you question the late Dr. Tiller's simple philosophy "trust women." To my mind, however, it shows how powerful that statement is, because it's a reminder that being pro-choice is simply about extending the same assumption to women that they're the best judges of their own lives that we extend to situations where men can be choice-makers. When we allow men to make their own personal life-choices, we may not always think, as outsiders, they're making the choice we consider the most ethical. But we believe trusting them is better than the alternative, which is allowing outsiders who don't know the situation as thoroughly to make those choices.
I see abortion as even less ethically fraught than divorce, because in divorce, people with real conscious brains feel real pain and drop real tears. If a man wakes up one day and decides, for no external reason, that he doesn't love his wife anymore, he will break her heart when he divorces her. But it's better than the alternative of a loveless marriage. If someone feels she can provide for one baby but not for two, I still feel she's in the best position to make that call. And the good news is there isn't a living, breathing person whose heart will break upon being not chosen.
This may be easier for me because I personally feel that the number of children that is best for me is zero, so I can easily see why someone thinks two is better than three. As an ethical question, I tend to think providing for existing children trumps bringing forth potential children. In my experience, most parents overestimate their abilities to provide as much time, energy, and resources as a child will need, so if someone truly balks at extending themselves to an extra, unexpected child, I tend to trust that judgment. The more kids you have, the more you have to spread resources around. The ethical argument in favor of not overextending yourself and potentially neglecting a child trumps ethical arguments about what you owe to people who don't even exist yet.