5Gardberg M,Laakkonen E,Salevaara M.Intrapartum sonography and persistent occiput posterior position:a study of 408 deliveries[J].Obstet Gynecol,1998,91(4):746-749.
6Senecal J,Xiong X,Fraser WD,et al.Effect of fetal position on second-stage duration and labor outcome[J].Obstet Gynecol,2005,105(4):763-772.