Let's agree that, from the moment of conception, we have a living thing -- and, if the parents are human, this living thing belongs to no other species than homo sapiens. So what? That fact doesn't in itself determine the moral status of the product of conception.

Here's one possible view: as the human zygote/embryo/foetus develops, its death becomes a more serious matter. At the very beginning, its death is of little consequence; as time goes on its death is a matter it becomes appropriate to be more concerned about.

In fact, that view seems to be exactly the one most of us take about the natural death of human zygotes/embryos/foetuses. After all, few of us are worried by the fact that a high proportion of conceptions spontaneously abort: few of us are scandalized if a woman who finds she is pregnant by mistake in a test one week after conception is pleased when she discovers that the pregnancy has naturally terminated a few days later. Similarly for accidental death: suppose a woman finds she is a week pregnant, goes cross-country horse riding, falls badly at a jump, and spontaneously aborts. That might be regrettable, but we wouldn't think she'd done something terrible by going riding and running the risk. (Compare: we do think it is a matter for moral concern that there are high levels of infant mortality in some countries; we would be scandalized by a woman celebrating the death of an unwanted newborn baby: we would be appalled at someone risking the life of nearly nine-months old foetus by going in for some potentially dangerous sports.)

So: our attitudes to the natural or accidental death of the products of conception seem to suggest that we regard them as of relatively lowly moral status at the beginning of their lives, and of greater moral standing as time passes. It would be consistent with such a view to take a similar line about unnatural deaths. For example, it would be consistent with that to think that using the morning-after pill is of no moral significance, while bringing about the death of an eight month foetus is getting on for as serious as killing a neonate, with a gradual increase in the seriousness of the killing in between.

Now, the point I'm making here isn't that this "gradualist" view is right (actually, I think it is, but you don't have to agree for present purposes). The point is that it that it isn't obvious that it is wrong. In other words, it isn't obvious that an all-or-nothing attitude to members of the species homo sapiens has to be right. It is not obvious that agreeing that the products of human conceptions are also human means that we should assign them all the moral weight we give to developed human beings. There's room for argument.