The fetal-pelvic index as an indicator of fetal-pelvic disproportion: a preliminary report.

The objective of this study was to develop a standardized method of identifying fetal-pelvic disproportion by comparing fetal head and abdomen circumferences with the respective maternal pelvic inlet and midpelvis circumferences. Fetal ultrasound and maternal x-ray pelvimetry measurements were performed on 75 term pregnant women with indications for a trial of labor. By means of individually measured fetal and maternal pelvic diameters, circumferences of the fetal head, fetal abdomen, maternal pelvic inlet, and maternal midpelvis were computed. Based on four circumference differences between the fetus and maternal pelvis (fetal head--maternal pelvic inlet, fetal head--maternal midpelvis, fetal abdomen--maternal pelvic inlet, and fetal abdomen--maternal midpelvis), a fetal pelvic index number was derived from the sum of the two most positive values. Of the 27 patients requiring cesarean sections for failure to progress in labor, all but four had positive fetal pelvic index values (sensitivity = 85%). Of the 48 patients who were delivered vaginally, all but four had negative fetal pelvic index values (specificity = 92%).