Correlation between blood flow impedance of the corpus luteum and serum progesterone concentrations. (a): Correlation between corpus luteum-resistance index (CL-RI) and serum progesterone concentrations (n = 46). (b): Receiver operating characteristic (ROC) curve analysis. CL-RI and serum progesterone concentrations were measured during the mid-luteal phase (6–8 days after ovulation). Serum progesterone concentrations were significantly and negatively correlated with CL-RI (p < 0.01, single regression analysis). ROC curve analysis was performed to determine the cutoff value of the CL-RI providing the best values of sensitivity and specificity for determination of normal luteal function and luteal phase defect. The cutoff value of 0.51 provided the best combination with 84.3% sensitivity and 85.6% specificity to discriminate between normal luteal function and luteal phase defect.