The Historical Basis of Anti-Abortionist Sentiment
If we put aside all metaphysical and religious dogma and rely only on facts, or objective truths, then it is impossible to arrive at a hard anti-abortion position. Historically, abortion was condemned in Christianity (and in its derivative Islam) because it was believed that ensoulment—the obtaining of a soul—takes place before birth. In Christianity, it was held that ensoulment must take place at conception otherwise the human nature of Christ would have been fashioned in the womb, apart from His divine soul, prior to ensoulment. Since orthodox Christian theology held that Christ was not a man who became God (or a God who indwelt in a man) but rather God incarnate (the God-Man), it was necessary to insist that ensoulment happened at the very moment of conception. Thus, ensoulment was believed to take place at conception. Consequently, any abortion would necessarily constitute murder.
Only baptized individuals were allowed into heaven, which is why the early Church insisted on baptizing infants. Abortion would damn an infant/fetus to hell for all eternity as the souls of the unborn were unable to be saved through baptism. Abortion was also frowned upon by early Christians because of its association with Temple prostitution. This is the religious background for why abortion was viewed as murder. Islam, lacking the concept of a God-Man, held that ensoulment occured four months into the pregnancy. Thus, abortion was regarded as somewhat permissible prior to ensoulment, but not permissible after. The thing to note about these justifications for opposing abortion is that they are anti-scientific. They are based on arbitrary religious dogma and not on objective fact.
Beyond these old religious dogmas, the main argument against abortion is that the fetus either is human or has the potential to become a fully-developed human. This, though, is also non-scientific and non-objective. It is based on a vague sentiment and usually also on the speciesist assumption that human life is somehow more valuable than all other forms of life. This sentimentality and speciesism that lies beneath anti-abortion sentiments is really, in my opinion, based upon subconscious religious assumptions that have become ingrained in us through cultural and social conditioning—we, knowingly or unknowingly, assume that we are the greatest of all creatures, alone created in the image and likeness of God.