Caton, D., Corry, M.P.,  Frigoletto, F.D., Hopkins, D.P., Lieberman, E., Mayberry, L., Rooks, J.P.,  Rosenfield, A., Sakala, C., Simkin, P., Young, D. (2002). The nature and management of labor pain. American Journal of Obstetrics & Gynecology, 186(5), S1-S15.