[1] Yokohama City Univ, Sch Med, Dept Obstet & Gynecol, Yokohama, Kanagawa 232, Japan
[2] Yokohama City Univ, Sch Med, Coll Nursing Care, Yokohama, Kanagawa 232, Japan
[3] Yokohama Natl Univ, Ctr Hlth Serv Sci, Yokohama, Kanagawa 240, Japan
[4] Yokohama City Univ, Int Coll Arts & Sci, Yokohama, Kanagawa 232, Japan
[5] Kanto Gakuin Univ, Sch Nursing, Yokohama, Kanagawa, Japan
[6] Yokohama City Univ Med, Dept Obstet & Gynecol, Yokohama, Kanagawa 2360004, Japan