The diagnostic accuracy of pelvic measurements: threshold values and fetal size

This study aimed to correlate pelvic dimensions and fetal size to the risk for cesarean section caused by protracted labor.

This is an observational, retrospective cohort study on pregnant women with an increased risk of labor dystocia. After pelvimetry, pelvic adequacy was clinically tested in a trial of labor. A multivariable regression analysis was made to identify the risk factors for cesarean section. Two subgroups were established according to the size of the estimated fetal head circumference (HC) (arbitrary cutoffs of ≤340 and >340 mm), and the pelvic measurements were compared by the mode of delivery. Receiver operating characteristic (ROC) curves were evaluated.

Altogether, 274 patients were ultimately included. The mean size of the maternal inlet was 1.0 cm larger in fetal HC >340 mm group compared with ≤340 mm. In the vaginal delivery group, the difference was 1.3 cm. In the multivariable modeling, maternal age (odds ratio [OR] 1.09, 95 % confidence interval [CI] 1.02-1.17), fetal HC (OR 1.05, 95 % CI 1.02-1.09), and maternal inlet circumference (OR 0.95, 95 % CI 0.92-0.97) had significance for the risk of cesarean section. In the ROC analysis, the area under the curve (AUC) value for the pelvic inlet was 0.736 (p < 0.001, 95 % CI 0.656-0.816), and in the subgroups with fetal HC ≤340 and >340 mm, AUCs were 0.634 (p < 0.11, 95 % CI 0.493-0.775) and 0.836 (p < 0.001, 95 % CI 0.751-0921), respectively.

Labor arrest was associated with the linear relationship between the maternal pelvic dimensions and the fetal size. Therefore, the approach should be changed from standardized pelvimetric threshold values to passenger-passageway ratio analyzed by multivariable modeling to find more accurate methods to predict cephalopelvic disproportion.