1. Department of Obstetrics and Gynecology, Cheil General Hospital & Women's Healthcare Center, Kwandong University College of Medicine, Seoul, Korea.
2. Department of Preventive Medicine, Eulji University Hospital, Eulji University, Daejeon, Korea.
3. Department of Obstetrics and Gynecology, Eulji University Hospital, Eulji University, Daejeon, Korea.
4. Department of Obstetrics and Gynecology, Eulji General Hospital, Eulji University, Seoul, Korea.
5. Department of Epidemiology, University of Michigan, School of Public Health, Ann Arbor, USA.