1Department of Obstetrics and Gynecology, College of Medicine, University of Ulsan, Asan Medical Center, Seoul, Korea. 2Department of Obstetrics and Gynecology, Korea Cancer Center Hospital, Seoul, Korea. 3Triton Diagnostics, Alameda, California, USA. 4Department of Obstetrics and Gynecology, Yale University School of Medicine, New Haven, Connecticut, USA.