Well, one set of reasons so many Christians are pro-life are not distinctively Christian. They're the reasons also used by secular pro-life groups (such as the excellent New Wave Feminists, Secular Pro-Life, and Feminists for Nonviolent Choices). No biological distinction can be drawn between an unborn human and a newborn one, except the degree of dependence upon the mother. They are both humans at a physical level, whether more or less developed. Another reason might be that abortion occurs most predominantly amongst the poor and disenfranchised, and is a silent form of societal cleansing; another would be that it is an economic and social decision made at the expense of those who cannot defend themselves or speak for themselves; whilst yet another might be that the constant claim that women cannot cope with being mothers in certain situations is destructive to women (New Wave Feminists have a wonderful piece of literature which emphasizes that to be a mother is to be a superhero, and no-one can tell you you can't be a superhero).
Then there's the “Bible quotes” category of reasons Christians are pro-life. Children in the Bible are seen as a blessing and as part of our human purpose. For instance, God tells humans in the beginning to “be fruitful and multiply” (Genesis 1.28); children are “a gift from the Lord; the fruit of the womb is a reward” (Psalm 127.3). God is also very involved in the lives of the unborn, according to the Bible – for instance, “you knit me in my mother's womb” (Psalm 139.13), “God . . . from my mother's womb had set me apart and called me through His grace”(Galatians 1.15), and many other instances. The Bible gives many specific instances of God's care for the unborn and His love for children.
However, I suspect that behind all of these, there is one big overarching reason so many Christians are pro-life. I can't answer for all Christians, but I know this is true for me and for many others. All those Bible verses, all that biological data, points the Christian to one big theological statement: that human life, even human life which does not seem worthwhile, is inherently valuable. Christianity has an anthropology which says: humans are amazing creatures, but they've corrupted themselves, and none can stand in front of God and claim innocence. However, God so loved the world that He sent His only Son to save it – and save it by submitting Himself to death on a cross. Next to Him on that Cross was a robber who called for Jesus' help and received it, despite being a convicted criminal. The great missionary of the early church, Paul, started out as an accessory to the murder of Christians. Slave traders and war criminals have turned to Christ and been accepted. And so have the meek, the poor, the vulnerable, who come into church equal and as loved as the rich and powerful, despite their lack of earthly status.
And if that applies to our moral and economic states, it surely applies to our medical states. God finds the Alzheimer's victim beautiful and well worth dying for – He prepares a place of wholeness and healing for people who we often find emotionally traumatic or a burden. God gives the quadriplegic the ability to pray world-changing prayers, and He gives the Motor Neurone Disease sufferer the ability to show Christ to the world in the midst of their awful sufferings. So perhaps the baby is being born into a poor neighbourhood. Perhaps they're coming at an inconvenient point for a career. Perhaps they won't be able to be valuable to the world. Perhaps, perhaps, perhaps. But in the face of each Perhaps God has a Certainly: Certainly I created them. Certainly I love them. Certainly I have died for them. Many Christians are pro-life not because they're misogynistic or scientifically ignorant or economically simple – they defend the rights of the unborn because they believe all humans are eternally valuable and precious, no matter what in earthly terms they have to offer.