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?

Objectives To compare the angle of progression on transperineal ultrasound imaging between different modes of delivery in prolonged second stage of labor with occipitoanterior fetal position. Methods We prospectively evaluated 41 women at term (≥ 37 weeks) with failure to progress in the second stage of labor. Only cases with occipitoanterior fetal position were included in the final analysis. These cases were classified into three groups: Cesarean section for failure to progress, vacuum extraction for failure to progress, and spontaneous delivery following prolonged second stage of labor. Transperineal ultrasound examination was performed just before digital examination and subsequent delivery. The angle between a line placed through the midline of the pubic symphysis and a line running from the inferior apex of the symphysis tangentially to the fetal skull (the so‐called ‘angle of progression’) was measured offline by an observer blinded to the mode of delivery. Results There were 26 cases with occipitoanterior fetal position (Cesarean section, n = 5; vacuum extraction, n = 16; spontaneous delivery, n = 5). Logistic regression analysis showed a strong relationship between the angle of progression and the need for Cesarean delivery (R2 measure of fit = 55%, likelihood ratio chi‐square P < 0.0001). When the angle of progression was 120°, the fitted probability of either an easy and successful vacuum extraction or spontaneous vaginal delivery was 90%. Conclusions This is the first report to document a strong relationship between an objective ultrasound marker (angle of progression) and the mode of delivery following prolonged second stage of labor with occipitoanterior fetal position. A predictive model using this parameter would allow better decision making regarding operative delivery for obstructed labor. Copyright © 2009 ISUOG. Published by John Wiley & Sons, Ltd.