As a theologian, I believe the issues of birth control and abortion are, at their most fundamental levels, not just issues of individual rights, but theological issues, and that support for either side must ultimately be presented in the form of a defensible theological argument.
If the option of pro-choice is to be a religious position, it will eventually have to be argued that there are times and cases when God demands an abortion. Not simply permits, not closes His or Her eyes to it, but demands it.
The fundamental position of both the Roman Catholic church and the pro-life movement is that the most important of all considerations is the brute fact of a single individual human life. Every single human life is considered sacred, even at conception: more sacred than any other consideration. It is the quantity of life that is being defended, not the quality of that life.
As the Christian writer Tertullian said eighteen centuries ago: "That is a man [sic] which will be a man: you have the fruit already in the seed." And if it is to become a person, then either to actively stop conception from taking place, as birth control does, or to actively terminate the development of that zygote and fetus, as abortion does, is seen as a sin against the wishes of God, and must be stopped. As a conservative theologian might put it, "God demands it" -- assuming they have this God-business right.
This is why those who think of themselves as pro-life have such zeal, fervor and such a deep commitment to stopping what they see as a murderous crime against not only the individual conceptions, fetuses and babies, but against God Himself. Let's look more closely at this.
The pro-life argument against abortion is easy to reduce to its inherent absurdity. If one human life is good, then two are better, a million are better yet, and the seven billion we have on the earth now are miracles of life to be welcomed and encouraged. But why stop with only seven billion? Why not seven trillion? The question on this worldwide scale is not when to stop population growth, but how it can ever be stopped.
On a smaller, more personal scale: if an individual life, in and of itself, is always good, no matter how many children the mother has had by what age, no matter how many are crowded into a single woman's life, a family's life, or the squalor of inner-city ghettos, then how could anyone committed to "pro-life" ever argue for birth control or abortion?
Ironically, the best theological model for abortion as a moral choice -- and often, a necessity -- comes from the Roman Catholic Church, in a papal encyclical called Rerum Novarum , written by Pope Leo XIII in 1891. It has been updated by the church three times, in 1931, 1961, (and 1991), to modernize the language and polish a few of the concepts. The title "Rerum Novarum" means "On New Things," which also fits its adaptation to today's issues of birth control and abortion.
This 30-page essay did more to change the social structures of the western world than the entire "Social Gospel Movement." It brought about revolutions in attitude that were absolutely fundamental in getting both churches and governments to change child labor laws and help establish workers' unions all over the world. And the encyclical had this power because it was, at bottom, a theological argument of the first order: in theological jargon, an argument about what God demands.