Comparison of different methods of measuring angle of progression in prediction of labor outcome

Objective: First, to compare the manual sagittal and para-sagittal and automated para-sagittal methods of measuring the angle of progression (AOP) by transperineal ultrasound during labor, and second, to develop models for the prediction of time-to-delivery and need for cesarean section (CS) for failure to progress (FTP) in a population of patients undergoing induction of labor.

Methods: This was a prospective observational study of transperineal ultrasound on a cohort of 512 women with singleton pregnancies undergiong induction of labor. A random selection of 50 stored images was assessed for inter- and intra-observer reliability between methods. In the cases of vaginal delivery univariate linear, multivariate linear and quantile regression were performed to predict time-to-delivery. Univariate and multivariate binomial logistic regression were performed to predict CS for FTP in the first stage of labor.

Results: The intra correlation coefficients (ICC) for the manual para-sagittal method for a single observer was 0.97 (CI 0.95-0.98) and for two observers was 0.96 (CI 0.93-0.98) indicating good reliability. The ICC for the sagittal method for a single observer was 0.93 (0.88-0.96) and for two observers was 0.74 (0.58-0.84) indicating moderate reliabilty for a single observer and poor reliability between two observers. Bland-Altman analysis demonstrated narrower limits of agreement for the manual para-saggittal approach than for the sagittal approach for both single and two observers. The automated para-sagittal method failed to capture an image in 19% of cases. The mean difference between sagittal and para-sagittal methods was 110. In pregnancies resulting in vaginal delivery, 54% of the variation in time-to-delivery was explained in a model combining parity, epidural and syntocinon use during labour and the sonographic findings of fetal head position and AOP. In the prediction of CS for FTP in the first stage of labour a model which combined maternal factors with the sonographic measurements of AOP and estimated fetal weight was superior to one utilising maternal factors alone (area under the curve 0.80 vs 0.76).

Conclusions: First, the method of measuring AOP with greatest reliability is the manual parasagittal technique and future research should focus on this technique, second, over half of the variation in time to vaginal delivery can be explained by a model that combines maternal factors, pregnancy characteristics and ultrasound findings, and third, the ability of AOP to provide clinically useful prediction CS for FTP in the first stage of labour is limited.