With human beings, you subtract accidental characteristics such as race, sex, and hair color until you are left with the things which cannot be subtracted without destroying humanness itself. One such characteristic is a rational faculty. At birth, the fetus is biologically autonomous and is a self-owner with full individual rights. As long as the fetus is physically within the woman's body, nourished by the food she eats, sustained by the air she breathes, dependent upon her circulatory and respiratory system, it cannot claim individual rights because it is not an individual. It is part of the woman's body and subject to her discretion.
Birth is the point at which the fetus becomes an actual human being in the legal sense of that term.
There is no point, other than conception, at which such a clear, objective change occurs in the status of the fetus. Antiabortionists often detail the physical development of the fetus, the development of toes and brainwaves, in order to give weight to the claim that it is human. But this development, by their own standards, is irrelevant, since they have already assigned individual rights to the zygote, which has no discernible features. Therefore these features are beside the point. One means by which antiabortionists attempt to load the issue of abortion against the woman and in favor of the fetus is by ascribing responsibility to the woman.