Persistent occiput posterior (OP) position occurs in approximately 5% of women in the second stage of labor and is a risk factor for labor complications. These complications include a 50% rate of labor arrest during the second stage, an over 60% rate of cesarean or instrumental vaginal delivery and an almost 20% incidence of obstetrical anal sphincter injuries. Persistent OP position is among the strongest risk factors for failed instrumental vaginal delivery and is also associated with increased frequency of neonatal complications, such as a 3% rate of neonatal acidemia, a 10% rate of neonatal morbidity and an 80% higher risk of birth trauma mostly due to failed operative delivery. Digital vaginal examination is considered the standard of care in the assessment of fetal position in labor. However, clinical examination has demonstrated limited accuracy in diagnosing occiput position, particularly in cases of none occiput anterior position and before performing an instrumental delivery.
Intrapartum sonography can be used to assist the assessment of the correct fetal head position and hence support clinical decision-making before considering or performing an operative delivery in the second stage of labor given its objectivity and reliability in the diagnosis of the head position. Thus, some international guidelines endorse the use of transabdominal sonography before instrumental delivery to confirm the fetal occiput position in case of uncertainty after vaginal examination. Furthermore, the assessment of the fetal head station with transperineal sonography has been shown to be more accurate than clinical evaluation in predicting the outcome of instrumental delivery.
There have been no data on the role of intrapartum sonography in predicting the outcome of instrumental vaginal delivery by vacuum extraction (VE) in fetuses with sonographically confirmed OP position. This study aimed to investigate the outcomes of VE in fetuses with sonographically confirmed OP position before the procedure.
Singleton pregnancies at term with sonographically confirmed fetal occiput posterior position before the vacuum extraction were enrolled in 3 academic maternity units. Fetal head station was assessed using transperineal sonography measuring the angle of progression and the head perineum distance. The primary outcome was failed vacuum extraction, defined as the need for cesarean delivery. Secondary outcomes included adverse maternal and/or adverse neonatal outcomes and complicated vacuum extraction, with the latter defined as failed vacuum extraction or at least 3 out of the following 6 parameters: 5-minute Apgar score <7, neonatal acidemia, admission to the neonatal intensive care unit, neonatal trauma, postpartum hemorrhage, and obstetrical anal sphincter injuries.
Among the 98 patients included in the study, vacuum extraction was successful in 94 (96%). Logistic regression analysis showed that the measurement of the head perineum distance was the only factor independently associated with failed vacuum extraction (odds ratio, 1.25; 95% confidence interval, 1.02-1.55; P=.03), with an area under the curve of 0.79 (P=.04). A head perineum distance cutoff value of 38.5 mm discriminated between successful and failed vacuum extraction, yielding a sensitivity of 75.0% (3/4), specificity of 84.0% (79/94), positive likelihood ratio of 4.7, and negative likelihood ratio of 0.3.
VE is successful in 95% of fetuses with OP position confirmed by ultrasound. Furthermore, the HPD measured by transperineal ultrasound plays a role, albeit modest, in predicting the outcome of VE, whereas AoP has no association with the success of the instrumental delivery. In practice, using a cutoff value of 38.5 mm, the HPD would identify 3 of the 4 cases of failed VE (sensitivity, 75%). However, it would incorrectly predict failure in 16 cases (specificity, 84%), which is 12 more than the actual number of failed VEs in the study group (n=4).
Study reported a high success rate of VE and low maternal and fetal complication rates in fetuses with sonographically diagnosed persistent OP position in the second stage of labor. Furthermore, authors demonstrate that HPD measured by transperineal ultrasound has a statistically significant but weak association with failed and complicated VE procedures, whereas AoP has no correlation with the success of the procedure. These data can be useful for intrapartum management of laboring women with fetuses diagnosed with OP position and requiring an instrumental delivery during the second stage of labor.
Source: Falcone V, Dall’Asta A, Romano A, et al. Vacuum extraction is successful in 95% of cases with an occiput posterior position: the results of a prospective, multicenter study. Am J Obstet Gynecol 2025;233:68.e1-12
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