1. The Degge Group, Ltd, Fairfax, VA, USA;
2. Department of Biomathematics and Biostatistics, Georgetown University, Washington, DC, USA;
3. Department of Epidemiology, University of Michigan, School of Public Health, Ann Arbor, MI, USA;
4. Department of Obstetrics and Gynecology, State University of New York at Buffalo, Buffalo, NY, USA