1. Department of Obstetrics and Gynecology, Kyung Hee University Hospital at Gangdong, College of Medicine, Kyung Hee University, Seoul, Korea.
2. Department of Obstetrics and Gynecology, Kyung Hee University Hospital, College of Medicine, Kyung Hee University, Seoul, Korea.
3. Department of Statistics, Sungkyunkwan University, Seoul, Korea.