Michael Tooley's justification of abortion rests upon a. a particular view of rights and b. a definition of personhood. Tooley argues that the desire presupposed by the right to life is the desire to continue as a being with psychological states.  Only a thinking self-conscious person with independent desires has such rights.  Here is a full text of this classic article, and here is an excellent summary of his views and those of other protagonists.

http://www.minerva.mic.ul.ie//vol2/bh.html

For a useful commentary laying out the logical structure of Tooley's argument, and a model of how we might take notes, go to:

http://www.philosophy.umd.edu/Faculty/pcarruthers/140-22.pdf