帝王切開分娩における真の胎児頭部下降の超音波評価

Accurate assessment of fetal head descent during labor is fundamental to the safe performance of operative vaginal delivery. However, conventional evaluation by digital vaginal examination is subjective and prone to inter-observer variability. Although intrapartum transperineal ultrasound has improved objectivity by quantifying the presenting part of the fetal head, these assessments may not reliably reflect the vertical level of the largest fetal head diameter, particularly in the presence of fetal head molding and caput succedaneum. The suprapubic descent angle was recently introduced as a transabdominal ultrasonographic parameter designed to address this limitation by indexing the position of the largest head diameter. Nevertheless, evidence regarding its external validity and reproducibility in independent clinical settings remains limited.

To externally validate the suprapubic descent angle as an ultrasonographic marker of fetal head descent in operative vaginal delivery and to assess its correlation with digital examination and established intrapartum ultrasound parameters.

This prospective cohort study was conducted at a tertiary perinatal center between January and December 2025. Women with singleton, term, cephalic pregnancies undergoing operative vaginal delivery were consecutively enrolled. Intrapartum ultrasound was performed immediately before delivery. The suprapubic descent angle was measured offline by examiners blinded to labor progress and delivery outcomes. Correlations between the suprapubic descent angle and other measures of fetal head descent were analyzed using appropriate correlation coefficients. Measurement reproducibility was assessed using intra- and inter-observer intraclass correlation coefficients and Bland-Altman analysis. Sensitivity analyses were performed in cases with occiput anterior fetal head position to evaluate the potential influence of rotation on measurement agreement.

A total of 191 women were included, yielding 375 ultrasound images for analysis. The suprapubic descent angle demonstrated strong inverse correlations with digital station (r = -0.85; 95% confidence interval, -0.83 to -0.88), angle of progression (r = -0.83; 95% confidence interval, -0.80 to -0.86), and transperineal ultrasound station (r = -0.85; 95% confidence interval, -0.82 to -0.88). Measurement reproducibility was excellent, with an intra-observer intraclass correlation coefficient of 0.985 (95% confidence interval, 0.930 to 0.977; p < 0.001) and an inter-observer intraclass correlation coefficient of 0.960 (95% confidence interval, 0.930 to 0.977; p < 0.001), accompanied by narrow limits of agreement. In sensitivity analyses restricted to occiput anterior cases, agreement further improved, indicating reduced measurement variability when fetal head malrotation was absent.

The suprapubic descent angle is a feasible, highly reproducible, and externally valid ultrasonographic measure of fetal head descent in operative vaginal delivery. By more closely reflecting the vertical level of the largest fetal head diameter, the suprapubic descent angle provides clinically complementary information to digital examination and transperineal ultrasound assessment. Incorporating this parameter into intrapartum evaluation may enhance risk stratification and support more informed decision-making during operative vaginal delivery by helping to identify potential overestimation of fetal head descent using conventional assessment methods. Outcome-focused studies are warranted to determine suprapubic descent angle-based thresholds that predict delivery success and procedure-related complications.