When I was an atheist, I eventually came to understand that the belief that saying that a new organism created by two humans wasn't itself human was essentially a euphemism I could use to keep myself sane while still arguing in favour of abortion on social harmony grounds. But it is difficult to lie to oneself.
At a minimum, it is logical to assume a human life biologically begins at the same time it ends ... using brain function. Any argument that a fetus with nothing but a brain stem, or a brain damaged adult with only a functioning brain stem remaining, is 'alive' is based on "sanctity of life" or other religious or pseudo religious beliefs.
As an atheist, I have no qualms or guilt about choosing to pull the plug on any biological "human" without a functioning cerebrum and cerebellum.
The third trimester is when the abortion debate gets at all tricky for me.
