Three-dimensional ultrasound in monitoring progression of labor: a reproducibility study

The aim of this study was to evaluate the intraobserver and interobserver reproducibility of measurements, obtained from stored ultrasound volumes, related to fetal head progression in the birth canal.

From January to September 2009, serial ultrasound volumes were obtained from women in labor, stored and separately evaluated by two different operators using the SonoVCAD™ labor software. Volumes were aligned using the pubic bone and the urethra as references. In the sagittal plane of the pelvis, the following data were calculated: head progression distance (the distance between the infrapubic line and the lowest part of the fetal skull), head direction (the angle between the infrapubic line and the major longitudinal axis of the fetal head) and head progression angle (the angle between the longitudinal axis of the pubic bone and a line joining the lowest edge of the pubis to the lowest convexity of the fetal skull). In the transverse plane, the angle formed by the midline with the anteroposterior axis of the maternal pelvis (midline angle) was also measured. For each measurement, intraobserver and interobserver reproducibility was determined.

We analyzed 30 ultrasound volumes. For all parameters, interobserver variation was significantly higher than intraobserver variation. Reproducibility was good for all parameters, except for the midline angle. Among the different ultrasound measurements, the progression angle presented the highest reproducibility.

Measurements obtained from stored ultrasound volumes to assess fetal head progression in the second stage of labor have good reproducibility, with progression distance and progression angle being the most reproducible parameters.