Perhaps the better question is: When is a developing human life valuable?
When you understand biochemistry/genetics/biology you stop seeing "life" a moment of creation, but rather a continuum of self-perpetuating biochemistry. If an embryo is considered a human life because it is a small bundle of cells with unique DNA, then the same "value" should be applied to a few skin cells that flake off every day. If it is about potential to become a fully functional human, then I would argue that I can take your skin cells and make genetic clones of you with current laboratory techniques.
Lets just call this entire debate what it really is. An argument of the value of human life, and everyone will have a different opinion. Religious people will tend to value human life from a spiritual perspective and beleive all human life should be cherished because its "Gods creation", but non-religious people have varying shades of grey. Is something alive and worth keeping once it has the potential to grow? Once it can feel? Once it takes on identifiable living traits (heart-beat)? To me the value of human life can be measured by its relationship with others, and in the womb that relationship is often only between the mother and father. If they dont care, its none of our business.