Measurement of fetal head descent using the „angle of progression“ on transperineal ultrasound imaging is reliable regardless of fetal head station or ultrasound expertise - Dückelmann, A M; Bamberg, C; Michae-lis, S A; Lange, J; Nonnenmacher, A; Dudenhausen, J W (Ultrasound Obstet Gynecol, 2010)
et al.