Objective: It has been shown that the modified myocardial performance index (Mod-MPI) is associated with higher reproducibility than conventional MPI because it uses mitral and aortic valve 'clicks' to calculate each time period. We aimed to construct normal reference values for the Mod-MPI after 19 weeks' gestation. Methods: The Mod-MPI was calculated in the left ventricle of 557 normal fetuses at 19-39 weeks' gestation. The isovolumetric contraction time (ICT), isovolumetric relaxation time (IRT) and ejection time (ET) were measured using the Doppler signals (clicks) of the opening and closing of the mitral and aortic valves as landmarks. Mod-MPI was calculated as (ICT + IRT)/ET. Results: Throughout gestation, there was a small increase in the Mod-MPI (at 19 weeks it was 0.35 ± 0.027 (mean ± SD); at 39 weeks, 0.37 ± 0.029; Mod-MPI = 0.33 + 0.001 x gestational age (GA) (weeks), r2 = 0.017). Of the three components, ICT remained constant, IRT increased (IRT = 0.028 + 2.52 x GA (weeks)), and ET slightly decreased (ET = 0.184 -3.65 x GA (weeks)). Fetal heart rate (FHR) had no effect on the Mod-MPI, but the duration of IRT, ICT and ET decreased by 13-15% when FHR increased from 130 to 160 beats per minute. Conclusion: The GA-adjusted reference values for the Mod-MPI and the three time periods used for its calculation can be applied to fetal cardiac evaluation in the presence of pregnancy-associated complications.