An adult is just a bunch of cells on a couch. A newborn baby is just a bunch of cells in a cradle. How big a bunch of cells does an individual have to have in order to be worthy of life?
Why 10? That's just your arbitrary number. Someone else will say 100, or 100,00, or a million. Or, like the noted professor Peter Singer, a year old baby if not developed up to his standard of what a human being should be.