I don't know if you are still working on this but you can also try looking at Henci Goer's books:
The Thinking Woman's Guide to a Better Birth
http://www.hencigoer.com/betterbirth/
& her other one:
Obstetric Myths Versus Research Realities
http://www.hencigoer.com/obmyth/
(the books include references to peer-reviewed studies)