Predicting mid-pelvic interspinous distance in women using height and pubic arch angle.

OBJECTIVE: To predict the interspinous distance (ISD) using the relationship between female height and pelvimetric measures on magnetic resonance (MR) images. METHODS: We obtained measurements of the pubic arch angle (PAA), inlet-anteroposterior (AP) distance, mid-pelvis AP distance, outlet-AP distance, ISD, and ischial tuberosity distance using 710 pelvic MR images from nonpregnant reproductive-aged (21-50 years) women from January 2014 to June 2020. Patient height was also assessed from medical records. We determined the formula for predicting ISD using multiple regression analysis. RESULTS: The mean ± standard deviation of the height, PAA, inlet-AP distance, mid-pelvis AP distance, outlet-AP distance, ISD, and ischial tuberosity distance were 160.0 ± 5.5 cm, 87.31 ± 6.6°, 129.7 ± 9.0 mm, 119.7 ± 8.5 mm, 111.71 ± 8.90 mm, 108.88 ± 8.0 mm, and 121.97 ± 11.8 mm, respectively. Two significant regression formulas for predicting ISD were identified as follows: ISD = 0.24973 × height - 0.06724 × inlet-AP distance + 0.12166 × outlet-AP distance + 0.29233 × ischial tuberosity distance + 0.32524 × PAA (P < 0.001, R2 = 0.9973 [adjusted R2 = 0.9973]) and ISD = 0.40935 × height + 0.49761 × PAA (P < 0.001, R2 = 0.9965 [adjusted R2 = 0.9965]). CONCLUSION: ISD is the best predictor of obstructed labor. This study predicted ISD with 99% explanatory power using only the height and PAA. The PAA can be measured by transperineal ultrasound. This formula may successfully predict vaginal delivery or cephalopelvic disproportion.