A fairly standard argument against the position that human life and personhood begin at conception, and therefore that the embryo is to be protected from the violence of abortion from conception onward, is to point out the great number of embryos that never implant in the womb and are simply flushed away by the woman's body. Estimates cited can be as high as 60%.
For a number of reasons this argument is a red herring. First, what occurs in nature beyond our control is not a guide to moral action. Just because tsunamis can wipe away thousands of souls in a day, does not mean that we do not protect those people from genocide. In ages past, the infant and child mortality rate was astronomical, but that did not prevent abortion from being condemned in Christendom. Second, these natural miscarriages happen before the woman even knows that conception has occurred, well before an abortion is even considered, so it has little relevance to the moral question of how to treat an implanted fetus. Perhaps it might be more relevant in a debate on the morning after pill.
Nevertheless, it raises the question of the worth of a human life in its earliest days, and introduces some uncertainty in the pro-life argument that human personhood begins at conception. This is especially true if one is religious: how could God expect us to value the unborn, if he himself designed pregnancy so that perhaps half of all children die in the first few days of life?
The idea that human personhood begins at conception is actually fairly recent. Medieval theologians and philosophers like Thomas Aquinas would have instead placed the point at which the embryo becomes a human person, the point at which it becomes ensouled, at the point of quickening, some 14 weeks into pregnancy. Such thinkers still condemned abortion prior to quickening, but did not regard it as the taking of a human life.
These thinkers, however, were relying on the embryology of their day. The early stages of embryonic development were not, so far as they could tell, "human" because they did not bear the human form. And since the human soul just is the form of a human body, a body that did not yet bear the human form, did not yet possess a human soul and was not a human person. These conclusions changed when we learned that the human form is already possessed by the embryo, from the moment of conception, in its DNA, and that the embryo is a distinct organism with it's own principle of growth and development leading, if not violently interrupted, to adulthood.
This leaves us with a great waste of human life built into nature, it seems. I would like to speculate on an alternative.
Perhaps these embryos, or at least some portion of them, fail to implant because they are not, in fact, human beings. Perhaps there are severe genetic or epigenetic deformities in the embryos that render them not healthy organisms but mere human tissue that would not, under any circumstance, develop into an adult. In this case the fertilization process was a failure and managed to produce only a kind of tumor derived from human cells, but not a human organism endowed with worth and inalienable rights. To put it in terms of the medieval thinkers, the embryo in question is human matter, but does not possess a human form; it is not ensouled.
I am not suggesting that human life therefore begins at implantation. That conclusion is not scientifically sound. Every human life still begins at conception, since conception creates a distinct human organism with human DNA and a human destiny, it is just that not all conceptions begin a human life. This would not, of course, justify the morning after pill, since that pill prevents healthy embryos from implanting. This solution avoids impugning God for his carelessness, and avoids making a mockery of the value of human life. And most importantly, it gives no ground to the abortionist, who enters the picture only after implantation. And anyway, we would have no way of knowing which embryos are human and which are not until we know whether they implant. We would be required to exercise caution in that case, so that no fertilized egg should be destroyed wantonly, for fear of destroying a human life.