My personal belief is that life begins at the moment the soul is present in the physical body.
I don't know when that is (biblical reference, anyone?) so I'll err on the side of life and say at conception.
'Err' is not the quite right term. It is the only position to take since noone has ever proven when the spirit takes up residence with the soul of life. Siding with life is never error since to not do so ends any possibility for correcting and error. I'm led strongly by scriptural teaching to believe the spirit is present as soon as organism life is evidenced. But I haven't a scientific proof of same ... yet.
When does the soul leave the body?