(to the author)I don't wish anything evil or harmful on you. I just hope a change takes place in your heart and you realize that life doesn't begin when you decide it does. No matter what the supreme court may have said in Roe. It begins at conception.
The phenomenon of twinning would allow for a reasonable counter-argument; how can it be said that a zygote is "a" human being when it might turn out to be two, three, or four (I think there have been identical quads, though I'm not sure)? If an embryo splits into two identical pieces, it doesn't seem right to say one of them was created when they split and the other was created at fertilization, but at the same time it's not reasonable to say that they were both created at fertilization when, had events following fertilization gone differently, there would never have have been a split and thus there would never have been more than one person.
I don't think there can be any doubt that the embryo/fetus becomes a full-fledged person many months prior to birth. That does not, however, mean that personhood goes all the way back to fertilization; there are other developmental milestones such as implantation which are just as clearly defined, and by which time the nature of the devloping blatocyst/embryo is much more clearly defined. While fertilization doesn't establish how many people there are going to be, I think that question is pretty well decided by the the time of implantation.
IMHO, just as building a house requires three ingredients (completed plans, construction materials, and job site) I would say that producing a human likewise requires sperm, egg, and development site (natural or simulated uterine lining; the latter having never been successfully used yet, but leaving myself open to the possibility).