1. Division of Statistical and Data Sciences, RTI International, Research Triangle Park, NC, USA;
2. Biostatistics and Epidemiology Division, RTI International, Research Triangle Park, NC, USA;
3. Department of Obstetrics & Gynecology, Northwestern University, Chicago, IL, USA;
4. Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, MD, USA;
5. Department of Obstetrics, Gynecology & Reproductive Sciences, University of Pittsburgh, Pittsburgh, PA, USA;
6. Department of Obstetrics & Gynecology, University of California at Irvine, Irvine, CA, USA;
7. Department of Obstetrics & Gynecology, Indiana University, Indianapolis, IN, USA;
8. Department of Obstetrics & Gynecology, University of Utah, Salt Lake City, UT, USA;
9. Department of Obstetrics & Gynecology, University of Pennsylvania, Philadelphia, PA, USA;
10. Department of Obstetrics & Gynecology, University of Texas Medical Branch-Galveston, Galveston, TX, USA;
11. Department of Obstetrics & Gynecology, Columbia University, New York, NY, USA;
12. Department of Obstetrics & Gynecology, Northwestern University, Chicago, IL, USA