Correlation between clinical and sonographic station during the second stage of labor: Which role for the occiput position? secondary analysis of a prospective dataset

Background: The assessment of the occiput position and station of the fetal head are required for managing the second stage of labor. Clinical assessment has shown limitations in terms of accuracy and reproducibility, and previous sonographic data have demonstrated a different pattern of head descent in the event of occiput posterior compared to non-occiput posterior position. Objective: To evaluate the relation between the position of the fetal occiput and the clinical and sonographic assessment of fetal head station during the second stage of labor. Methods: Single center prospective observational study including singleton pregnancies in the second stage of labor. Firstly, eligible cases underwent vaginal examination for the assessment of fetal head position and station. Secondly, transabdominal and transperineal "research scan" was performed by a second operator blinded to the findings of the vaginal examination to ascertain the occiput position and evaluate the sonographic station by measuring the angle-of-progression (AoP) and the head-perineum distance (HPD). The primary endpoint was the comparison between occiput posterior (OP) and non-OP fetuses in terms of agreement rate between digital examination and ultrasound assessment and diagnosis of head engagement vs non-engagement. Results: 303 women were included, of whom 81 (26.7%) had sonographically confirmed OP position. The rate of agreement between clinical and ultrasound assessment showed no difference between the cases in non-OP and those in OP position. Non-OP fetuses showed a higher rate of clinical underestimation (127/222, 57% vs 31/81, 38%, p < 0.01); conversely, in fetuses in OP position a higher frequency of clinical overestimation was observed (22/81, 27%, vs 29/222, 13%, p < 0.01). Paired comparison of the clinical and sonographic station showed a narrower AoP for the clinical stations 0, +1 and +2 [120 (94-144) degrees vs 129 (106-161) degrees, p < 0.01; 133 (116-145) degrees vs 138 (109-169), p = 0.02; 140 (120-165) degrees vs 149 (130-181) , p < 0.01, respectively] and a longer HPD for the clinical stations 0 and +1 [38 (33-55) vs 36 (25-46) mm, p = 0.01 and 35 (30-44) vs 32 (20-46) mm, p = 0.03, respectively] in fetuses in OP position. Conclusions: OP position is associated with a higher rate of clinical overestimation of the head station if compared to non-OP position and with less favorable sonographic indicators of head station for the clinical stations 0, +1 and +2.