The reason religion is so powerful

[Neo-Scholastic]

If the cells from your cheeks can be transformed into totipotent stem cells (and have whatever other zygote-like qualities are necessary), then yes, without question that is the beginning of a new organism. Its analogous to the hydra, in which any cell in its body is totipotent and can produce a new hydra organism.

Granting that, I still don't see how it gets us to forcing women to bear children against their will. No one is arguing that a fertilized egg isn't biologically human with the potential to eventually develop into a person, the argument is that there is no evidence that it is a person. An acorn is biologically an oak, but an acorn is not an oak tree. A bird egg is not a bird. How is an embryo a baby?

Aren't you raising the bar? If human rights are grounded in one's humanity, then one must simply be human to have them. If instead personhood is the criteria for having human rights then there is a subset of humans that do not actually have human rights.