Is the natural rate of death of early-stage human beings high? Almost certainly, it is much higher than the natural rate of death for, say, five-year-olds. In a significant percentage of what appear to be spontaneous abortions, are there really no human organisms dying? Probably. Would accounting for this likelihood bring down our estimate of the death rate? Yes, probably. But we don’t seem to have good numbers. Do pro-lifers (including the pope) treat non-embryos as though they were embryos? No. Are they in any way logically committed to doing so? No. These are pretty much the results that I suggested at the beginning.
Two notes here, to stave off confusion: First, I think any argument that a high natural death rate for a group would somehow justify the deliberate killing of members of the group is irremediably faulty (and that the faulty inference cannot be rescued by relabeling it an intuition). Even a very high rate would not, I think, get you any closer to a justification. The question of what the rate is, while interesting for other reasons, strikes me as largely irrelevant to the moral question.
Second, it’s important to distinguish conceptually between disabled and severely disabled embryos, on the one hand, and biological entities that aren’t embryos on the other. An organism that, e.g., integrates its own functioning and has an active internal disposition to develop but has some defect that prevents it from being able to implant would, I think, qualify as a severely disabled embryo but still as an embryo.