1. Department of Obstetrics and Gynecology, Seoul National University College of Medicine, Seoul, Korea.
2. Department of Obstetrics and Gynecology, Seoul St. Mary's Hospital, College of Medicine, The Catholic University of Korea, Seoul, Korea.
3. Department of Biostatistics, Seoul Metropolitan Government Seoul National University Boramae Medical Center, Seoul, Korea.
4. Department of Obstetrics and Gynecology, Soonchunhyang University Seoul Hospital, Seoul, Korea.
5. Department of Obstetrics and Gynecology, Seoul National University Hospital, Seoul, Korea.