1. Department of Obstetrics and Gynecology, MedStar Washington Hospital Center, Washington, District of Columbia
2. Department of Obstetrics and Gynecology, MedStar Georgetown University Hospital, Washington, District of Columbia
3. Department of Biostatistics and Epidemiology, MedStar Health Research Institute, Hyattsville, Maryland
4. Department of Obstetrics and Gynecology, Northwestern University Feinberg School of Medicine, Chicago, Illinois