1. Department of Obstetrics and Gynecology, Hallym University Dongtan Sacred Heart Hospital, Hwaseong, Korea.
2. Department of Obstetrics and Gynecology, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea.
3. Department of Obstetrics and Gynecology, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea.
4. Department of Biostatistics, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea.