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Purpose: To determine the validity of intrapartum ultrasound (IPUS), and particularly the angle of progression (AOP), in predicting delivery mode when measured in real-life clinical practice among women with protracted second stages of labor. Methods: Using electronic medical records, nulliparous women with a second stage of labor of ≥ 3 h ("prolonged") and a documented AOP measurement during the second stage were identified. The ability of a single AOP measurement in "prolonged" second stage to predict a vaginal delivery (VD) was assessed. Fetal head descent, measured by AOP change/h (calculated from serial measurements), was compared between women who delivered vaginally and those who had a cesarean delivery (CD) for arrest of descent. Results: Of the 191 women who met the inclusion criteria, 62 (32.5%) delivered spontaneously, 96 (50.2%) had a vacuum extraction (VE) and 33 (17.3%) had a CD. The mean AOP was wider among women who had VD (spontaneous or VE) compared to those who had CD (153° ± 19 vs. 133° ± 17, p < 0.001). Wider AOPs were associated with higher rates of VD and an AOP ≥ 127° was associated with a VD rate of 88.6% (148/167). Among the 87 women who had more than one AOP measurement, the mean AOP change per hour was higher in the VD group than in the CD group (15.1° ± 11.4° vs. 6.2° ± 6.3°, p < 0.001). Conclusion: Ultrasound-assessed fetal head station in nulliparous women with a protracted second stage of labor can be an accurate and objective additive tool in predicting the mode and interval time to delivery in real-life clinical practice. [ABSTRACT FROM AUTHOR]