Personally, the simplest explanation for when a human life begins seems like when all the basic components of what goes into a life come together, which would be conception.
Before then, you need something extra for an organism that could become a human life in any sense would form. After then, it's just another developmental stage of an organism that's already formed.
Obviously not all people share that definition, but I'm not really sympathetic to that argument honestly. Not all people share my belief that post-birth abortions shouldn't be allowed either; Peter Singer doesn't and he has some arguments that in a purely utilitarian sense would be valid for it, yet I believe the law should prevent killing newborn infants. Fundamentally, government is where we hash out different value-based conceptions like these on a societal level - since this is mostly a question of when life begins, I think people should advocate for what they believe in, rather than adopting an "I believe X, you believe Y, so it shouldn't be legislated on" stance.
I'm more sympathetic to arguments that the protection of human life by the strictest definition (i.e. the one I'm using) has to be balanced, which seems totally valid to me. If the life of the mother is in danger for example, she should be able to abort. Really, that's injecting some utilitarian calculus into an otherwise hard "life should be protected" stance in areas where it gets messy: the mother has more feeling than the unborn child.