Predicting the difficulty of operative vaginal delivery by ultrasound measurement of fetal head station

Background: Clinical assessment of fetal head station is difficult and subjective; it is mandatory before attempting operative vaginal delivery.

Objective: The principal objective of our study was to assess whether measurement of the perineum-to-skull ultrasound distance was predictive of a difficult operative vaginal delivery. Secondary objectives included evaluation of the interobserver reproducibility of perineum-to-skull ultrasound distance and comparison of this measurement and digital examination in predicting a difficult operative delivery.

Study design: This was a prospective cohort study including all cases of operative vaginal deliveries in singleton pregnancies in cephalic presentation >34 weeks' gestation, from 2012 through 2015. All data were entered prospectively in a medical record system specially devised to meet the requirements of this study.

Results: Of the 659 patients in whom perineum-to-skull ultrasound distance was measured prior to operative vaginal delivery, 120 (18%) met the composite criterion for a difficult extraction. Perineum-to-skull ultrasound distance measurement of ≥40 mm was significantly associated with the occurrence of a difficult extraction based on the composite criterion, after adjustment for parity, presentation type, and fetal macrosomia (odds ratio, 2.38; 95% confidence interval, 1.51-3.74; P = .0002). The intraclass correlation coefficient between the perineum-to-skull ultrasound distance measured by the first operator and that measured by the second operator was 0.96 (95% confidence interval, 0.95-0.97; P < .0001). Based on the receiver operating characteristic curve analyses, perineum-to-skull ultrasound distance was a more accurate predictor of difficult operative delivery than digital vaginal examination (P = .036).

Conclusion: Measurement of the perineum-fetal skull ultrasound distance is a reproducible and predictive index of the difficulty of instrumental extraction. Ultrasound is a useful supplementary tool to the usual clinical findings.