[1] Hokkaido Univ Hosp, Matern & Perinatal Care Ctr, Sapporo, Hokkaido, Japan
[2] Hokkaido Univ, Ctr Environm & Hlth Sci, Sapporo, Hokkaido, Japan
[3] Asahikawa Med Univ, Dept Social Med, Asahikawa, Hokkaido, Japan
[4] Japanese Red Cross Hokkaido Coll Nursing, Fac Nursing, Kitami, Hokkaido, Japan
[5] Asahikawa Med Univ, Dept Obstet & Gynecol, Asahikawa, Hokkaido, Japan
[6] Sapporo Med Univ, Sch Med, Dept Obstet & Gynecol, Sapporo, Hokkaido, Japan