1. Department of Obstetrics and Gynecology, Gil Hospital, Gachon University College of Medicine, Incheon, Korea.
2. Department of Obstetrics and Gynecology, Severance Hospital, Yonsei University College of Medicine, Seoul, Korea.
3. Institute of Women's Life Medical Science, Seoul, Korea.
4. Biostatistics Collaboration Unit, Yonsei University College of Medicine, Seoul, Korea.