Law cannot define define when life begins.
Life is a continuum. A living sperm and living ovum combine to form a living zygote; there is never a time when anything is not alive. The nervous system begins to form at about 3 weeks; at this point, one can say that the embryo begins to be aware. There is *no* beginning point at which one can say that something unalive has become alive. Scientifically, it just isn't there. And it is ludicrous to claim that one can define such a point with human laws.
The legal question to me should hinge not on conception but on implantation. Human life begins at conception because it can begin at no other point in time. However, pregnancy begins at implantation. Abortion ends pregnancy by killing the fetus. That tells me all I need to know about abortion to know it is evil, and when a person has an abortion to end a pregnancy, they have killed a human being.
A simple positive pregnancy test is all that is needed because abortions aren't performed on people who are not pregnant. Case closed.