The issue is that we use a combination of science, ethics, philosophy, religion, etc etc to try and get to that point. It becomes an exercise in de conflicting where the acceptable limit is.A more relevant question in light of medical advances would be "at what point does the life of an unborn child become of sufficient value to warrant protecting?" If your answer is not until birth or as one respondent put it "at crowning" then we as a society are on a slippery slope that inevitably leads to racism and discrimination because I of course am of more value to society than you because I am (fill in the blank for your favourite group)
One such exercise is using brain death as a metric. When does higher brain function end leading to when does higher brain function start. It’s the same sort of argument for or against assisted suicide.