Well, yeah, changing the definition of "full human rights" status in such a major way would create a huge thicket of legal issues. That's why we have lawyers and judges and cases to sort these things out. The law does not exist in some philosophical vacuum, it has to function in the real world. Going back to the opening post: the obvious answer would be to base citizenship on birth, not conception. The moment when ovum meets sperm is just too damn hard to prove.

Just because a blastocyst has the protections of a full human being does not mean that every other right and responsibility must be conferred at the same moment. Children are protected by many laws, but we don't let them vote. Does this make sense to you?