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BACKGROUND To date no research has focused on the sonographic quantification of the degree of flexion of the fetal head in relation to the labor outcome in women with protracted active phase of labor. OBJECTIVE To assess the relationship between the transabdominal sonographic indices of fetal head flexion and the mode of delivery in women with protracted active phase of labor. STUDY DESIGN Prospective evaluation of women with protracted active phase of labor recruited across three tertiary maternity units. Eligible cases were submitted to transabdominal ultrasound for the evaluation of the fetal head position and flexion, which was measured by means of the occiput-spine angle (OSA) in fetuses in non-occiput posterior (OP) position and by means of the chin-to-chest angle (CCA) in fetuses in OP position. The OSA and the CCA were compared between women who had vaginal delivery vs those who had cesarean delivery. Cases where obstetric intervention was performed solely based on suspected fetal distress were excluded. RESULTS 129 women were included, of whom 43 (33.3%) had OP position. Spontaneous vaginal delivery, instrumental delivery and cesarean delivery were recorded in 66 (51.2%), 17 (13.1%) and 46 (35.7%) cases, respectively. A wider OSA was measured in women who had vaginal delivery compared to those submitted to cesarean delivery due to labor dystocia (126+14 vs 115+24, p<0.01). At ROC curve the area-under-the-curve (AUC) was 0.675, 95%CI (0.538-0.812), p<0.01, and the optimal OSA cut-off value discriminating between cases of vaginal delivery vs those delivered by cesarean delivery was 109 degrees. A narrower CCA was measured in cases who had vaginal delivery compared to those undergoing cesarean delivery (27+33 vs 56+28 degrees, p<0.01). The AUC of the CCA in relation to the mode of delivery was 0.758, 95%CI (0.612-0.904), p<0.01, and the optimal cut-off value discriminating between vaginal delivery and cesarean delivery was 33.0 degrees. CONCLUSIONS In women with protracted active phase of labor, the sonographic demonstration of fetal head deflexion in OP and in non-OP fetuses is associated with an increased incidence of cesarean delivery due to labor dystocia. Such findings suggest that intrapartum ultrasound may contribute in the categorization of the etiology of labor dystocia.