Objective. To examine how well ultrasound-assessed occipitoposterior (OP) position or high sagittal (HS) position in primiparous women with a prolonged first stage of labor predicts a vaginal delivery and the duration of labor. Design. Prospective observational study. Setting. Stavanger University Hospital, a secondary referral center in Norway. Population. 105 primiparous women with prolonged first stage of labor. Methods. Ultrasound assessment of fetal head position. Main outcome measures. Vaginal delivery vs. cesarean section and duration of labor. Results. Twenty-five fetuses (24%) were delivered with cesarean section (CS), 45 (43%) had an operative vaginal delivery and 35 (33%) delivered spontaneously. Eleven (27%) of 41 fetuses in OP position at the time of inclusion were born in OP position. Ten (24%) of the 41 fetuses in OP position at inclusion were delivered with CS compared with 15/64 (23%) fetuses in other positions (p= 0.91). Twenty-eight fetuses were in sagittal position and 12 in HS position, assessed with ultrasound at the time of diagnosed prolonged labor. Seven (58%) of 12 in HS position delivered vaginally and five (42%) had a CS (p= 0.89). Time from inclusion to labor was not significant longer either for fetuses in OP compared with non-OP positions or for fetuses in HS compared with non-HS positions. Conclusions. Most fetuses in OP or HS positions in the first stage of labor will rotate spontaneously and have a high probability of being delivered vaginally.