Optimizing forceps delivery safety: predicting anal sphincter injury with intrapartum ultrasound

To assess whether intrapartum ultrasound parameters can predict obstetric anal sphincter injury (OASI) in forceps deliveries.

This retrospective cohort study included women undergoing forceps delivery for live, singleton, term cephalic pregnancies at Showa Medical University Hospital, between April 2021 and September 2024. Ultrasound was performed immediately before forceps application to evaluate angle of progression (AoP), head direction (HD), transperineal ultrasound (TUS) station, and midline angle (MLA) at rest and during pushing. Delta (Δ) values were calculated as differences between pushing and resting. OASI was diagnosed clinically and via transanal ultrasound. Parameters were compared between women with and without OASI. The Benjamini-Hochberg procedure corrected for multiple testing, with significance below < 0.05.

Among 256 women, 23 (8.9%) developed OASI. Maternal, labor, and neonatal characteristics were similar between groups. Several intrapartum ultrasound parameters were significantly associated with OASI: TUS station during pushing (median 3.2 and 4.2, p = 0.002, q = 0.012), ΔTUS station (1.1 and 1.6, p = 0.006, q = 0.042), HD at rest (28° and 38°, p < 0.0001 q < 0.001), HD during pushing (32° and 50°, p < 0.0001, q < 0.001), and ΔHD (3.2° and 10°, p = 0.0005, q = 0.004). HD during pushing showed the best predictive performance (AUC = 0.871), with an optimal cutoff of 36° (sensitivity 0.69, specificity 0.90).

Intrapartum ultrasound, particularly HD during pushing, is a valuable predictor of OASI in forceps deliveries. A cut-off value of 36° may identify women at higher risk and improve forceps delivery safety.