That’s not scientific. One cannot determine if a human spirit resides in a blastocyst or not by scientific methods, so to assume something based on a set of observations (no circulatory or central nervous systems have formed yet, but will, for example) is also not scientific.
I do not consider what a pre-implantation blastocyst *might* become at some point in the future—I only consider its present characteristics. A person really only exists within a functional brain; without a living brain, there simply is no person.
Science does not and cannot answer questions of the soul. I would say that the Bible does not truly answer that, either. There was a time when the soul was considered to appear at the time of quickening—about 3 to 5 months into pregnancy, when the mother feels movement for the first time. And we now know that the embryo moves long before the mother feels it. You could just as well deem birth the point at which the soul appears, and justify it biblically.
What do you call other fertilized ova that eventually flush out of a woman's body during menstruation because they didn't implant?
-PJ