In the debates about the morality of abortion, there are usually two self-titled "sides": pro-choice and pro-life. I believe these are, for the most part, misnomers, but also signifying something deeper about each side of the argument and the constituent belief systems supporting them.
Christianity is the primary proponent of pro-life, particularly the Catholic Church, one of the few groups in the world that is opposed to abortion in absolutely all circumstances. The Church's stance on abortion is merely an extension of its fundamental affirmation of life itself, exemplified morally by its stance on birth control, homosexuality, premarital sex, murder, etc. Pro-life for Catholicism is not simply a political movement - it is the expression of a fundamental and necessary worldview within the Church and her Tradition, upholding the inherent value of Creation apart from suffering and sin, and upholding objective morality apart from justification by pain, convenience or perceived "fairness". No difficulty, "cause" or excuse can justify sin.
Most adherents of the pro-choice movement use that term to signify that espoused view that abortion is morally permissible (or at least justified) because the mother has the right to choose for herself with "her own body", as they often say. This is an extension and mirror of the Supreme Court's decision to legalize abortion, itself founded on the argument of the mother's privacy. But in truth, the term pro-choice evidences an attribute of abortion itself.
In reality, the relationship between a mother and her unborn baby is a "pro-choice" arrangement. As a bond between a guardian and a dependant, with the dependant entirely relying on the guardian for life, health and wellness, the mother has the option and power to sever that bond. This applies to all guardian-dependant relationships, exemplified by other actions such as adoption, where the parents completely break their role as provider of the helpless child. But abortion being a pro-choice situation does not give equal validity to both choices of whether to provide for and sustain the dependant unborn baby, or break that connection. Like all situations involving human free will, abortion is pro-choice - but as in all other situations, one choice is wrong, and the other is right.
Having an individual be fully dependant on another person calls that person to love, responsibility, suffering and self-sacrifice for their wellbeing. Even more than in any other kind of relationship, this dependant bond brings to the forefront the centrality of interpersonal, communal responsibility, being "my brother's keeper", even when it inconveniences and even hurts us. The bond of mother and child is the highest expression of this.
If a pregnancy results from a sinful act, such as rape or incest; if it endangers the health or life of mother or baby; if it could involve future suffering of the mother from emotional, financial or social problems, or suffering for a deformed, retarded or unplanned-for baby - nothing can remove the inherent dignity and inalienable value of human life. We can be confused - very easily, in fact. Our culpability can be lessened. But abortion is wrong, and as St. Paul said, we are "without excuse" (Romans 2:1). The law of God is written in our hearts - all have a conscience. How can we not know that murdering an unborn child is wrong?
Or, perhaps a better question would be: how can anything make that murder permissible? The cure for pain, sin and evil is love - we must fight evil with good, not evil. Murder will not remove the taint of rape or incest, poverty or deformity, and there will never be a time when a parent feels absolutely, certainly comfortable with their situation to have a baby. Different excuses have been employed throughout history by to either avoid having children or to terminate those children after conception. But they are just that - excuses. Making abortion illegal is important, yes. But legality does not force abortion - it is chosen from the heart. Only a changed heart can change the world.