1. Department of Obstetrics and Gynecology and Center for Interdisciplinary Research in Women's Health, University of Texas Medical Branch, Galveston, Texas, USA.
2. Department of Obstetrics and Gynecology, George Washington University, Washington, District of Columbia, USA.
3. Office of Biostatistics, Department of Preventive Medicine and Community Health, University of Texas Medical Branch, Galveston, Texas, USA.