ACOG Practice Bulletin: Obstetric Analgesia and Anesthesia. Number 36, July 2002.

Gibbs, R. B. Karlan, A. Haney, I. Nygaard. Danforth’s Obstetrics and Gynecology. 9th ed. Lippincott Williams and Wilkins, 2008. Accessed on February 1, 2009 from http://www.ovidsp.tx.ovid.com.

Cunningham, F., K. Leveno, S. Bloom, J. Hauth, et al. Williams Obstetrics. 23nd ed. The Mcgraw Hill Companies, Inc. 2008. Accessed December 26, 2010 from http://www.accessmedicine.com.

Lockwood, C. Guidelines for Perinatal Care. 6th ed. AAP and ACOG. 2007.