The birth canal: correlation between the pubic arch angle, the interspinous diameter, and the obstetrical conjugate:...

Background: Assessment of pelvic configuration is an important factor in the prediction of a successful vaginal birth. However, manual evaluation of the pelvis is practically a vanishing art, and imaging techniques are not available as a real-time bed-side tool. Unlike the obstetrical conjugate diameter (OC) and inter spinous diameter (ISD), the pubic arch angle (PAA) can be easily measured by transperineal ultrasound. Objectives: Three-dimensional computed tomography bone reconstructions were used to measure the three main birth canal diameters, evaluate the correlation between them, and establish the normal reference range for the inlet, mid-, and pelvic outlet. Study design: Measurements of the PAA, obstetric conjugate (OC), and ISD were performed offline using three-dimensional post processing reconstruction in bone algorithm application of the pelvis on examinations performed for suspected renal colic in nonpregnant reproductive age woman. The mean of two measurements was used for statistical analysis which included reproducibility of measurements, regression curve estimation between PAA, OC, and ISD, and calculation of the respective reference range centiles for each PAA degree. Results: Two hundred ninety-eight women comprised the study group. The mean ± SD of the PAA, ISD, and OC were 104.9° (±7.4), 103.8 mm (±7.3), and 129.9 mm (±8.3), respectively. The intra- and interobserver agreement defined by the intraclass correlation coefficient (ICC) was excellent for all parameters (range 0.905-0.993). A significant positive correlation was found between PAA and ISD and between PAA and OCD (Pearson's correlation = 0.373 (p < .001), and 0.163 (p = .022), respectively). The best regression formula was found with quadratic regression for inter spinous diameter (ISD): 34.122778 + (0.962182*PAA - 0.002830*PAA²), and linear regression for obstetric conjugate (OC): 110.638397 + 0.183156*PAA. Modeled mean, SD, and reference centiles of the ISD and OCD were calculated using the above regression models as function of the PAA. Conclusions: We report significant correlation between the three pelvic landmarks with greatest impact on the prediction of a successful vaginal delivery: the PAA which is easily measured sonographically and the ISD and OC which are not measurable by ultrasound. This correlation may serve as a basis for future studies to assess its utility and prognostic value for a safe vaginal delivery.