Given that reasonable people disagree whether abortion is murder, how can someone who truly examines their own opinion fail to choose either 'life begins at conception' or abortion should be legal in all cases? While it is conceptually difficult to rationalize that a zygote or embryo is a human life and should be afforded all rights due, it is equally difficult and, in fact likely, more abhorrent to say that human life doesn't begin until a fetus exits and is severed physically from the mother's body. Isn't any choice other than one of these two a complete failure to have any true belief?
One way to approach this question is to think about what sorts of properties are morally significant. Some properties--such as the capacity to suffer or feel pain--might belong to a developing human life at some stages, but not others. A zygote probably doesn't feel pain, but it's plausible that an eight month old fetus does.