The angle of progression is the angle between a straight line drawn along the longitudinal axis of the pubic bone and a line drawn from the inferior edge of the pubic bone to the leading edge of the fetal cranium.
Data from: Kalache KD, Dückelmann AM, Michaelis SA, et al. 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? Ultrasound Obstet Gynecol 2009; 33:326.