1. Department of Obstetrics and Gynecology, Tomakomai City Hospital, Tomakomai, Hokkaido 053‑8567, Japan
2. Department of Obstetrics and Gynecology, Hakodate Central General Hospital, Hakodate, Hokkaido 040‑8585, Japan
3. Department of Pathology, Hokkaido University Graduate School of Medicine, Sapporo, Hokkaido 060‑8638, Japan