You can take that series pretty far before there is something resembling a human. I am not aware of any religious anti-abortion advocates who accept the notion that one, two, four, eight, sixteen celled clumps are NOT yet human. It is clearly a religious/soul view -- not an ethical/scientifically grounded view.
Do you know how early the fetus is beyond 1,2,3 or even a 1,000 cells? We are talking hours after conception (before you wake up the next morning!). Your argument may have validity for embryonic stem cell research, but not abortion. By the 3rd week of pregnancy the Fetus is developed to the point of recognizable human features. It has a working heart, brain and spinal cord. It feels pain and sucks it's thumb. And at three weeks most mothers have no idea they are expecting. Do you know when most abortions occur? Long after the multi-cell stage. We cannot kill this child just to ease the life of the mother, that just is not a good enough reason. We would never kill another segment of the population for the reasoning use to kill babies.