Vacuum extraction is successful in 95% of cases with an occiput posterior position: the results of a prospective, multi-center study

Background: Occiput-posterior (OP) position is associated with labor arrest, need of operative delivery and failed instrumental vaginal delivery with resulting adverse peripartum outcomes. Vacuum extraction (VE) is the most commonly performed type of instrumental delivery worldwide. This study aimed to investigate the outcome of VE in fetuses with sonographically confirmed OP position prior to the procedure.

Methods: Singleton pregnancies at term with sonographically confirmed fetal OP position prior to the vacuum extraction were enrolled in three academic maternity units. Fetal head station was assessed using transperineal sonography measuring the angle of progression (AoP) and the head-perineum distance (HPD). The primary outcome was failed VE, defined as the need for cesarean delivery. Secondary outcomes included adverse maternal and/or adverse neonatal outcomes and complicated VE, the latter defined as failed VE or at least three out of six parameters - Apgar <7 at 5 minutes, neonatal acidemia, admission to the neonatal intensive care unit, neonatal trauma, postpartum hemorrhage, and obstetric anal sphincter injuries.

Results: Of 98 patients included in the study, VE was successful in 94 (96%). Logistic regression analysis showed that the measurement of the head-perineum distance (HPD) only was independently associated with failed VE (OR 1.25, 95% CI (1.02-1.55), p=0.03) with an area-under the curve was of 0.79, (p=0.04). A HPD cut-off value of 38.5 mm discriminated between successful and failed VE, yielding a sensitivity of 75.0% (3/4), specificity of 84.0% (79/94), LHR+ of 4.7 , LHR- of 0.3.

Conclusions: Vacuum extraction (VE) is successful in 95% of fetuses with occiput posterior position confirmed at ultrasound. The head-perineum distance (HPD) measured at transperineal ultrasound has a significant albeit weak association with the outcome of VE.