I think that you are probably going to have to compose this on your own. Yet, you probably could get many insights on how to write this here. I think that a good introduction on the topic would briefly address why legal reform on the issue or even legal statements on the issue have been so challenging. This would embrace the idea of how the Constitutional issues of federalism, in terms of state and federal government differentiation on the topic, and personal choice have have helped to confuse and complicate the issue. At the same time, I think that the introduction might want to talk about how the issue has become so politicized that legal reform is very difficult to accomplish. Discussion of it being a "litmus test" or "hot potato" issue might be relevant here. I think that the introduction should probably close with what you intend on proving in the course of the paper on the topic. Along these lines, your introduction should embrace a good understanding and feel of what is going to be discussed in the paper. In the end, this might be why you would have to compose this. You alone understand what your paper is going to address and prove. The reader is going to be led to this conclusion and your introduction would be a start on such a quest.
google "roe v. wade" and read that first.