Breech progression angle: new feasible and reliable transperineal ultrasound parameter for assessment of fetal breech descent in birth canal

Objective: To assess the feasibility and reliability of transperineal ultrasound in the assessment of fetal breech descent in the birth canal, by measuring the breech progression angle (BPA). Methods: Women with a singleton pregnancy with the fetus in breech presentation between 34 and 41 weeks' gestation were recruited. Transperineal ultrasound images were acquired in the midsagittal view for each woman, twice by one operator and once by another. Each operator measured the BPA after anonymization of the transperineal ultrasound images. BPA was defined as the angle between a line running along the long axis of the pubic symphysis and another line extending from the most inferior portion of the pubic symphysis tangentially to the lowest recognizable fetal part in the maternal pelvis. Each operator was blinded to all other measurements performed for each woman. Intra- and interobserver reproducibility of BPA measurement was evaluated using the intraclass correlation coefficient (ICC). To investigate the presence of any bias, intra- and interobserver agreement was also analyzed using Bland–Altman analysis. Student's t-test and Levene's W0 test were used to investigate whether a number of different clinical factors had an effect on systematic differences and homogeneity, respectively, between BPA measurements. Results: Overall, 44 women were included in the analysis. BPA was measured successfully by both operators on all images. Both intra- and interobserver agreement analyses showed excellent reproducibility in BPA measurement, with ICCs of 0.88 (95% CI, 0.80–0.93) and 0.83 (95% CI, 0.71–0.90), respectively. The mean difference between measurements was 0.4° (95% CI, −1.4 to 2.2°) for intraobserver repeatability and −0.4° (95% CI, −2.6 to 1.8°) for interobserver repeatability. The upper limits of agreement were 12.0° (95% CI, 8.9–15.1°) and 13.6° (95% CI, 9.9–17.3°) for intra- and interobserver repeatability, respectively. The lower limits of agreement were −11.2° (95% CI, −14.3 to −8.1°) and −14.4° (95% CI, −18.2 to −10.7°) for intra- and interobserver repeatability, respectively. No systematic difference between BPA measurements was found on either intra- or interobserver agreement analysis. None of the clinical factors examined (maternal body mass index, maternal age, gestational age at the ultrasound scan and parity) showed a statistically significant effect on intra- or interobserver reliability. Conclusions: BPA represents a new feasible and highly reproducible measurement for the evaluation of fetal breech descent in the birth canal. Future studies assessing its usefulness in the prediction of successful external cephalic version and breech vaginal delivery are needed. © 2021 International Society of Ultrasound in Obstetrics and Gynecology.