1. Department of Epidemiology, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA;
2. Department of Health Behavior and Policy, Virginia Commonwealth University, Richmond, VA, USA;
3. Department of Health, Behavior and Society, Johns Hopkins University, Baltimore, MD, USA;
4. Department of Biological Sciences, North Carolina State University, Raleigh, NC, USA;
5. Department of Obstetrics and Gynecology, Duke University School of Medicine, Durham, NC, USA