Objective: To establish normal reference intervals of the fetal left modified myocardial performance index (MPI) with the use of stringent criteria for delimitation of the time periods. Study Design: A cohort of consecutive singleton fetuses was created including at least 20 fetuses for each completed week of gestation between 11 and 41 weeks. The isovolumetric contraction time (ICT), isovolumetric relaxation time (IRT), and ejection time (ET) were calculated using the clicks of the mitral and aortic valves as landmarks, and the MPI was calculated as follows: (ICT + IRT)/ET. Normal reference ranges for the MPI and its individual components were constructed by means of regression analysis of the mean and standard deviation against gestational age (GA). Results: A total of 730 fetuses were included. After a natural logarithmic transformation, a third degree cubic polynomial model (loge mean MPI = 0.0477 × GA – 0.002565 × GA2 + 0.000043 × GA3 –1.22, with GA measured in weeks) was selected to fit our data. There was a progressive increase in the mean MPI from 11 weeks (mean, 0.39; 95th centile, 0.51) to 41 weeks (mean, 0.55; 95th centile, 0.78) of gestation. While the mean ICT and IRT values increased with GA from 25 to 32 ms and from 39 to 51 ms, respectively, the ET showed an initial increase until 30 weeks and a progressive decrease thereafter. Conclusion: Normative references of left modified MPI from 11 to 41 weeks of gestation are provided, which could be useful in the assessment of cardiac function in fetuses.