If we judge this question in terms of the potential to become human life under certain predefined circumstances then we regard the very first instant that a zygote comes into being as "human life", and the woman who mistakenly has that one drink too many at a party the following night and blights a potential future human life becomes unknowingly guilty of manslaughter. In the same way we also judge that any dna samples taken that could be grown into a cloned human life must be, or else they too have lost their chance to live. Every finger, toe, ear and limb has its own right to life independent of the host body - and so does every cancer.
My own view is that a fetus becomes a little human at the point when it is capable of surviving surviving outside of its mother's body - or, as recent studies indicate, from around 22 weeks. Before that stage the possible future infant is no more or less than a (benign) growth in the mother's body, and is no excuse to deprive her of her own personal liberties and right to control her own body. Legally, any democratic government has no business infringing on the personal liberties of any of its citizens insofar as they do not endanger the existence of other blameless citizens.
Morally, I find the notion of abortion in most cases objectionable to say the least and I cannot imagine making that choice in my own personal life (though I also believe in some cases it can be the lesser of two wrongs). I also believe that government has no place in legislating morality.
The correct path for the pro-life movement is not to ban their fellow citizens from making choices they disagree with, but to reason with and educate them towards making the choices they support.