The cephalopelvic disproportion index. Combined fetal sonography and x-ray pelvimetry for early detection of cephalopelvic disproportion.

The smallest pelvic diameter (either the anteroposterior of the inlet or the bispinal of the midpelvis) was determined with x-ray pelvimetry and compared to the biparietal diameter of the fetal head at term as determined with sonography. The difference between the two indicates how much wider the smallest diameter of the bony pelvis is than the fetal skull and was termed the cephalopelvic disproportion index. Vaginal delivery was impossible when the index was less than 9 mm and impossible or very difficult when between 9 and 12 mm. When it was greater than or equal to 13 mm, 26% needed a cesarean section, 19% had a difficult vaginal delivery, and the rest delivered vaginally with minimal or no difficulty. This technique clearly indicates when a vaginal delivery is impossible (index less than 9 mm) or very difficult (index less than 13 mm). The specificity was 100%. The index therefore can recognize, before labor, the cases of obvious cephalopelvic disproportion that contraindicate a trial of labor. It does not indicate, however, if a vaginal delivery is possible in the setting of a high index (sensitivity, 51%) because of the interference of other factors besides the cephalic and pelvic bony dimensions considered here. The index may prove most important in determining if a vaginal birth should occur after a cesarean section because it can clearly identify some patients who need a repeat cesarean section.