As an atheist and materialist I do not accept the existence of a soul or spirit. I do not believe that at the moment of conception either of these objects descend upon a fertilized egg. I also believe that the question of when life begins is more of a red herring which typically assumes the pre-existence of one or both of these.
However, while I cannot accept that life begins at conception, I also cannot accept that it begins when a fetus passes through the birth canal. For me there is no qualitative difference between a fetus one minute before birth and a baby one minute after. Just as fertilizing an egg does not imbue personhood, neither does cutting the umbilical cord or passing through the birth canal. So this brings me back to the red herring, asking at what point can we say this is a living thing and its termination is murder.
The closest thing I have heard to an objective mark for calling a fetus life comes from an argument my wife put forth. She argued that since we have clear legal definitions of death (i.e. absence of brain activity and/or heartbeat) we can use these details in our definition of life. When sufficient brain activity is detectable, in other words it contains patterns not consistent with brain death, the fetus is alive and cannot be aborted.
This argument’s strongest point is that it is legally consistent. It does however raise practical problems. We can either draw an arbitrary line and say at a certain date an average fetus has the brain activity consistent with our definition of life or we can have a test run to attempt to measure brain activity. The first is a problem simply because it is arbitrary while the second would be totally unmanageable. Who would pay for the tests? How could we know that the doctors actually performed them? Are we really going to burden the legal system bringing doctors in to prove they gave this test? I couldn’t accept that.
So here I am, a snail crawling along the edge of a straight razor. Do I give in to my libertarian ideas and let people decide for themselves or do I protest on moral grounds arguing that at some point a fetus becomes a person and neither birth nor conception are adequate in determining this? This is my dilemma.