Williams obstetrics study guide, c2011: electronic t.p. (Barbara L. Hoffman, MD, Department of Obstetrics and Gynecology, University of Texas Southwestern Medical Center at Dallas)

Southwestern Medical Center website, accessed July 25, 2011 faculty & administration (Barbara L. Hoffman, M.D., Associate Professor, Obstetrics and Gynecology Southwestern Medical School)

Biblioteca del congreso (Estados Unidos), 11 septiembre 2017: catálogo electrónico https://goo.gl/PyRsmX