Atheists don’t believe in souls. They don’t believe God breathes live into a baby at birth.
Whatever life a baby has it has at conception. Basic 8th grade biology shows that a human life starts at conception. No life is added later. No scientist has ever witnessed a non living unborn turn into a human being at any stage other than conception.
Even Evolution shows the unborn is a human being at conception. Where did the zygote get her genes for her eye color? She got about 50% from her father and about 50% from her mother.