ACOG Practice Bulletin: Ultrasonography in Pregnancy, Number 101, February 2009.
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, L. Gilstrap, K. Wenstrom. Williams Obstetrics. 22nd ed. The Mcgraw Hill Companies, Inc. 2008. Accessed February 1, 2009 from http://www.accessmedicine.com.
Lockwood, C. Guidelines for Perinatal Care. 6th ed. AAP and ACOG. 2007.