1. Departments of Obstetrics and Gynecology, University of North Carolina School of Medicine Chapel Hill North Carolina USA
2. Department of Epidemiology UNC Gillings School of Public Health, University of North Carolina at Chapel Hill Chapel Hill North Carolina USA
3. Department of Obstetrics and Gynecology Duke University School of Medicine Durham North Carolina USA
4. Department of Microbiology and Immunology, University of North Carolina School of Medicine Chapel Hill North Carolina USA