We are not insects that go through a metamorphosis which has on one side a caterpiller and on the other side a butterfly. Human development is a seamless day-by-day progression to what is inarguably a human person. I understand your arguement, and once believed in it, but have come to the conclusion that, in the absence of certainty, the only human choice is to chose life.
It would certainly be morally wrong for someone to kill without justification something which 'may or may not' be a human being. On the other hand, should not the state, in order to charge someone with murder, have to prove that what was killed was actually a human being?
I don't think you'd have any trouble finding a jury of 12 people who would regard a 39-week fetus as a human being. Indeed, even without abusive voir dire one would have good luck picking such a jury at random. On the other hand, finding a jury which would regard a 1-week fetus as a human being would be another story. I don't think the state would be able to make a case to a jury except by excessively abusing the voir dire process.