Not an easy question, but in my opinion the spirit or soul does not exist or begin to function without the capability to make a decision... and that can be primal (voluntary muscle movement or primal brain survival thinking, i.e. cold, hunger, etc. vs. involuntary reflex) or conscious.
Does that exist in a embryo or fetus? At what point in development does that decision-making capability come into existence?
My own thinking is that there is no existence from a spiritual perspective until that moment. Can a stem cell feel and react to pain (primal thinking, decision-making... "must move because there is pain")? Can an embryo? At what point does the developing fetus have this brain function?
Every animal has this point in development. There is a reason God constructed the brain as it is, it is His tool and it is meant to begin functioning in this way, at some certain moment. I think that's where life actually begins, spiritually.