George on the Octuplets

The story of the single mom of octuplets (plus six other kids) becomes more disturbing every day.  One challenge for MoJ types, I think, is to inject a pro-life voice into the conversation that avoids the two extremes of either suggesting that parents should never have more than 2.5 kids or uncritically applauding any decision to bring new lives into the world.  In that regard, I was pleased to see that CNN interviewed Robby George as part of its coverage.  One snippet jumped out at me, though:

George said that, based on the information available, his personal ethical decision would probably support the woman's choice to carry all the babies to term. But he said that selective reduction is not the same as traditional abortion because the goal is the healthiest possible birth rather than the termination of a pregnancy.

"The babies didn't put themselves there; it's not their fault," George said. "There does seem to be a serious ethical question about killing one or more of them, even for the sake of maternal health."  

Are there circumstances under which selective reduction would be morally permissible?  If so, what sort of showing of endangerment would be required?