The blogmistress at worshipping at the altar of mediocrity has posted her thoughts on why she is pro-choice, along with links to some other current discussions of this question.
One of the reasons she gives for being pro-choice is "[pregnancy] is nine months of changing your lifestyle, your future, the risk of having to leave school or a job, the risk of being abandoned, abused, and/or alienated by your family, etc."
It is too true that many people and institutions in our society are deeply punitive about and hostile toward certain pregnancies. In a more enlightened age, young women will understand that they are buying into their own oppression when they bow to these pressures.
I don't get involved in pro-life/pro-choice debates much. I am more interested in how people react to individual abortions--their own or those of people they care about.
Often the political debate is a not-very-useful proxy for the real discussions people need to be having.
I know a couple that used to talk about abortion with each other purely as a political issue. I heard them do this on several occasions. She appeared to be mildly pro-life and he appeared to be mildly pro-choice. I found out from her after some years that they had had an abortion together early in their marriage and they had never talked about that abortion. They could only talk about "abortion in general". Meanwhile, she had come to believe that in the abortion, they had killed their child. She didn't feel safe saying this to him. The only way she could sort of suggest to him that she was not at ease about that abortion, that it had started to haunt her, was to express mildly pro-life views when the subject of abortion came up in group discussion at parties.