Transperineal ultrasound imaging in prolonged second stage of labor with occipitoanterior presenting fetuses: how well does the ‘angle of progression’ predict the mode of delivery? - Kalache, K D; Dückelmann, A M; Michaelis, S A; Lange, J; Cichon, G; Dudenhausen, J W (Ultrasound Obstet Gynecol, 2009)