1. Department of Obstetrics and Gynecology, Yonsei University College of Medicine, Seoul, Republic of Korea
2. Department of Health Insurance Research, National Health Insurance Service, Ilsan Hospital, Goyang, Republic of Korea
3. Department of Epidemiology and Biostatistics, Graduate School of Public Health, Seoul National University, Seoul, Republic of Korea
4. Department of Obstetrics and Gynecology, Ilsan Cha Hospital, Goyang, Republic of Korea