The crazy thing is that because this is a 'spectrum' as you aptly described it, attempts to define life along it are arbitrary almost by definition. The least and perhaps only non-arbitrary place to define the beginning of a human life is at conception as I see it. This rankles people in the pro-choice camp, but probably rankles lifers more when the consequence of that view is that i feel you should still be able to kill it. So really the conversation to me is about how we value human life at different stages, and the question of where does life begin is secondary and kind of misses the point in the context of not holding the view that all human life is objectively valuable or sacred. The whole problem is made worse by the fact that there is no right answer but rather a cultural spectrum of ethical stances...which only makes sense.