I understand your point, though the cells at conception form a zygote and not a fetus as minor as that point might seem. I think that the issue is that a fetus contains the requisite components of being a person. A zygote doesn't even come close and the best that can be argued, as I have, is that the zygote might contain a soul or consciousness that we cannot measure. Minus that and the zygote is just a few random cells. I understand what it might become... but we know that a woman is a woman with consciousness and a fully developed independent body and a zygote is a couple of cells that have a great chance of simply being purged from the system during the next female cycle of in a miscarriage.
I can see that some might call killing a zygote murder, but that only makes sense from a religious or moral perspective that probably has no logical or observable factors assigned to it.