1. Department of Perinatology and Gynecology; Kagawa University Graduate School of Medicine; 1750-1 Ikenobe Miki Kagawa 761-0793 Japan
2. Department of Ultrasound; Hiroshima Branch, GE Healthcare Japan, Nishihara; Asaminami-Ku Hiroshima Japan
3. Department of Obstetrics, General Perinatal Medical Center; Shikoku Medical Center for Children and Adults; Zentsuji City Kagawa Japan