Abstract Objective: To use intrapartum sonography as a tool to investigate the development of the persistent occiput posterior position during labor, as well as to identify parameters correlating with the outcome of labor. Methods: A prospective study of 408 women in labor after 37 weeks’ gestation with a singleton fetus in a vertex position using sonography at the onset of labor was performed. Fetal position, placental location, and maternal BMI (body mass index) were recorded. Outcome of labor was monitored for all relevant parameters. Results: Most (68%) of persistent occiput posterior positions develop through a malrotation during labor from an initially occipitoanterior position. Only 32% of persistent cases were occipitoposterior (dorsoposterior) at the onset of labor; operative interventions were required in 87.5% of these. Of the 61 (15%) occipitoposterior positions at the onset of labor, 53 (87%) rotated into an occiput anterior position. Persistent occiput posterior position was more common in the initially occipitoposterior group ( P < 0.01, Fisher exact test), and posterior placental locations were fewer ( z test, P = 0.05). Also, operative deliveries were more common in the group remaining occipitoposterior throughout labor ( P < .01, Fisher exact test). A higher maternal BMI correlated with neonatal weight ( P < .01, Pearson correlation), an increase in operative deliveries ( P = .032, Pearson correlation), lower Apgar scores at 1 minute ( P = .02, Spearman correlation), and increase in posterior placental locations ( P = .037, two-tailed t test). Conclusion: In most cases, persistent occiput posterior position develops through a malrotation and only in a little more than one-third of cases through absence of rotation from an initially occipitoposterior position. Higher maternal BMI correlates with higher fetal weight, increased operative deliveries, lower Apgar scores at 1 minute, and posterior placental locations. Intrapartum sonography proved to be useful in investigating the development of the persistent occipitoposterior position.