VP45.05: The ratio between fetal head circumference and maternal mid pelvic circumference: a simple index for cephalopelvic disproportion evaluation

Cephalopelvic disproportion (CPD) is the disproportion between the fetal head and maternal bony pelvis. The aim of the present study was a proof of concept evaluation of the ratio between fetal HC to maternal pelvic circumference (PC) as a predictor of CPD. Of 11,822 deliveries, 111 cases that underwent an abdominopelvic CT for any medical indication were included in the study retrospectively. They were divided into three groups: normal vaginal deliveries (NVD) (n = 84), instrumental deliveries (ID) (n = 7) and CD (n = 20) due to CPD. The correlation between CPD and Cephalopelvic Circumference Index (CPCI), which represents the ratio between the HC and PC in percent (HC/PC *100) was evaluated. The transverse mid pelvis (MP) diameter and mid PC were significantly smaller in ID and CS groups as compared to the NVD group: 9.5±1.1 and 9.8 ±0.9 vs. 10.4±1.0 cm (p = 0.002) and 32.1±2.6 and 33.5±3.6 vs. 34.8±2.3 cm (p = 0.034) respectively. The neonatal HC was larger in ID and CD groups as compared to the NVD group: 34.9±1.2 and 34.9±2.5 vs. 33.8±1.7 cm, respectively (p = 0.03). The MP cephalopelvic circumference index (MP-CPCI) was larger in ID and CD groups as compared to the NVD group: 109±9 and 103±11 vs. 97±8% respectively (P = 0.0003). In logistic regression analysis, the MP-CPCI was found to be an independent risk factor for CS due to CPD: each 1% increase in MP-CPCI increased the likelihood of CD for CPD by 11% (aOR 1.11, CI 95% 1.03-1.19, p=0.004). The adjusted odds ratio for CD due to CPD rose as the MP-CPD increased: 3.56 (CI 0.95, 1.01-12.6), 5.6 (CI 0.95, 1.63-19.45), 21.44 (CI 0.95, 3.05-150.84) and 28.88 (CI 0.95, 2.3-362.27) in MP-CPCI of 100, 105, 110 and 115, respectively. The MP-CPCI is a simple tool which can potentially distinguish between parturients at lower and higher risk of CPD. Prospective randomised studies are required to evaluate the feasibility of prenatal pelvimetry and MP-CPCI to predict the risk of CPD during labour.