In this study, we investigated the association between measures (the obstetrical conjugate and transverse diameter of pelvic inlet) of X-ray pelvimetry and cesarean delivery for labor dystocia using data from 913 women in labor. Our data indicated that the obstetrical conjugate was an independent variable correlating inversely with the incidence of C/S for dystocia but that maternal height could be a comparable variable to the conjugate in terms of the discrimination of the risk of C/S.
X-ray pelvimetry is a useful measure to evaluate the inlet to exclude CPD or contracted inlet and is recommended prior to labor induction/augmentation to negate the contracted inlet according to a guideline of maternal-fetal medicine in Japan (1). Thus, we have routinely performed pelvimetry on all women who are to undergo induction or augmentation for various reasons. However, among 913 women, contracted inlet was only observed in four women (4/913 = 0.4%). Conversely, a Cochran systematic review demonstrated that women who receive X-ray pelvimetry are likely to undergo C/S. Dystocia is not only one of the major reasons for C/S but is also associated with pelvic disproportion, which is evaluated by pelvimetry. Then, we took an interest in the clinical question of whether measurements obtained from X-ray pelvimetry are associated with the incidence of C/S and influence our decision of mode of delivery in the cases of dystocia. Our database of more than one thousand measures obtained from routine X-ray pelvimetry was valuable to clarify this question. The Cochran review performed in 2017 identified five studies with a total of 1159 women, and these studies compared X-ray pelvimetry (n = 582) versus no pelvimetry (n = 577) (2). The number of women who received X-ray pelvimetry in our study was greater than that in the Cochran review.
In this study, we excluded women undergoing C/S due to nonreassuring fetal status, fetal presentation or prior uterine operation from the subject of analysis. All of the remaining 913 women received X-ray pelvimetry prior to labor induction and/or augmentation and delivered a baby by either cesarean or vaginal deliveries. Overall, 37 women with C/S for dystocia and 876 women with vaginal delivery were compared. Univariate analysis revealed that maternal height and age, infant weight and head size, and the obstetrical conjugate were associated with risk of C/S, with significant differences. Regarding measures of X-ray pelvimetry, a significant difference (p < 0.01) was found in the obstetrical conjugate, but not the transverse diameter. These obstetrical variables are likely to confound each other; thereby, multivariate analysis was needed to identify an independent variable. Multivariate analysis revealed that the obstetrical conjugate was the only independent variable associated with C/S for dystocia. Taken together with data from the 2017 Cochran review, measures of the obstetrical conjugate might influence us to make decisions of mode of delivery. In our data, although the obstetrical conjugate identified a woman with contracted inlet and eight women with relatively contracted inlet, only two of them underwent C/S, suggesting that the obstetrical conjugate was not involved in the decision of mode of delivery. In other words, our statistical analysis indicates that a larger obstetrical conjugate resulted in a reduction of risk of C/S for dystocia (Odds ratio = 0.32).
Previous studies (6, 7) and a systematic review (2) have demonstrated that X-ray pelvimetry does not improve perinatal outcome and that routine X-ray pelvimetry is not useful to women with normal presentation. Regarding women with previous C/S or breech presentation, some studies have reported the usefulness of X-ray pelvimetry (8, 9). Our study did not address the usefulness of X-ray pelvimetry from the viewpoint of perinatal outcome, and thereby, we did not compare the outcome with the no-X-ray pelvimetry group.
In this study, the transverse diameter identified three cases of contracted inlet and one relatively contracted inlet, and all underwent C/S with CPD, while the obstetrical conjugate identified only two women with C/S among nine with contracted or relatively contracted inlet. The transverse diameter was more significant than the obstetrical conjugate in terms of the diagnostic measure of contracted inlet. Nevertheless, the abnormality detected by X-ray pelvimetry rarely occurred in our population (contracted inlet: 4/913, relatively contracted: 9/913, and CPD: 7/913 women). Several studies have reported that maternal height with or without a measure of pelvimetry can be a variable to predict dystocia (10) or CPD (11). Maternal height is also reported to correlate to the obstetrical conjugate (12). Indeed, our data clearly indicated the correlation between maternal height and the obstetrical conjugate, regardless of mode of delivery. Since the obstetrical conjugate was associated with risk of C/S for dystocia, maternal height in our population was thought to be an alternative variable for the conjugate. Our ROC analysis demonstrated the hypothesis that maternal height is comparable to the obstetrical conjugate to discriminate maternal risk of C/S for dystocia. Numerous previous studies have demonstrated that maternal height is inversely related to risk of not only C/S (13–17) but also labor dystocia (18–21). Taken together with our ROC curve, maternal short stature is sufficient to be an independent variable associated with the decision of mode of delivery.
In contrast, the transverse diameter did not show high diagnostic capacity. Interestingly, the 11.4 cm cut-off value for the transverse diameter on the ROC curve was concordant with the upper criterion (11.5 cm) of a relatively contracted inlet, while 11.7 cm for the obstetrical conjugate was quite different from the upper criterion (10.5 cm) of a relatively contracted inlet.