Intrapartum transperineal ultrasound as a predictor of instrumentation difficulty with vacuum-assisted delivery in primiparous women.

Objectives: To assess the capability of different intrapartum transperineal ultrasound parameters to predict the difficulty of vacuum extraction. This is a prospective observational study performed between 04/2012 and 03/2013 on 72 primiparous-women, ≥37-weeks with singleton pregnancies at full dilatation that underwent transperineal ultrasound before vacuum placement for foetal extraction. Working in a transperineal longitudinal plane we evaluated: progression-angle, progression-distance and head direction; in a transverse plane: midline-angle and head-perineum distance. The vacuum extractions were classified as easy-group (EG) (≤3 vacuum pulls), difficult/impossible-group (DG)(≥4 pulls). Occiput-posterior presentations were not assessed.

Results: Fifty-two (52) patients were studied (26 patients per study group). No differences were observed in obstetric, neonatal or intrapartum characteristics between the study groups, with the following exceptions: new-born (NB) weight (3147 g versus 3540 g) and the number of vacuum pulls (1.4 EG versus 4.3 DG; p < 0.0005). The progression angle was 133.1° (123°-143°) in EG and 109.2° (97.2°-121.2°) in DG (p < 0.0005); up direction of foetal head was 88% versus 34.5% (p < 0.0005); progression distance was 37 mm (26.6-47.4) versus 29.9 mm (8.8-51; p = 0.003); midline angle was 35° (15.4°-54.6°) versus 59.7° (34.5°-84.9°; p = 0.0005); head-perineum distance was 41.9 mm (35.2-48.6) versus 48.9 mm (40.5-57.3; p = 0.017). The area under the Receiver Operating Characteristic (ROC) curve for the progression angle was 0.9 (95%CI, 0.82-0.99), and the midline angle was 0.8 (95%CI, 0.67-0.92).

Conclusion: If previous to the placement of the vacuum cup the progression angle is ≤120°, the foetal head direction is horizontal or down, and the midline angle is ≥35°, there is an 85% chance that the delivery will require more than 4 vacuum pulls.