This is a disputed question, which is why I suspect you’re asking. Philosophers, theologians, pastors, politicians, doctors, lawyers, and many, many others have possible answers to this question. I’m feeling more medical than theological today, so I’ll suggest you start with the Blackwell Companion to Bioethics. To really answer the question, you have to answer what “human” means, and what “life” means. bit.ly/bcl-blackwell-bioethics