There is no magic line to cross where a group of cells becomes human life.  There is no magic point in time where an old person becomes senile or dead.  It is a continuum, sometimes extended as in birth, or in some forms of death, where a series of small changes over time transforms life into being or death into reality.   Where the arbitrary line is placed costs lives, there is no way around it.  We have to be wise and see the nuances of our choices to cause the least amount of harm.