There are increasingly IVF fetuses, thus, it is very beneficial for patients to indicate whether the fetal cardiac function is abnormal or not by non-invasive methods. The ejection fraction (EF) and ratios of E wave/A wave peak velocities (E/A) commonly used in clinic are only the evaluation of systolic function or diastolic function. However, the systolic and diastolic function influence each other when the blood flow index of fetal heart changes, so it is more accurate to evaluate the overall function of fetal heart. Therefore, it is highly essential to determine the reference range of RMPI.
Although the age and HR in the IVF group were higher than those in the SP group, there was no significant difference in RMPI between the two groups, which reflected that the age and HR had no significant effect on RMPI. In this study, linear regression analysis was used to show that there was no strong correlation between RMPI and GA, and between RMPI and HR in SP fetuses and IVF fetuses (Figs. 2). Ghawi et al. reported that MPI is independent from GA and HR . They pointed out that systolic ventricular function changes from high- to low-level, while diastolic ventricular function is gradually matured from low- to high-level with the increase of GA . MPI is advantageous for comprehensive evaluation of cardiac systolic and diastolic ventricular functions, therefore, this may be the reason why MPI does not change with GA.
The method of automatic measurement of RMPI used in the present study is time-saving, eliminating the need to measure ICT and IRT time, respectively. This is also essential to measure the spectrum of tricuspid valve and pulmonary valve, put the two pictures together, and then select the appropriate cardiac cycle (CC), and the system will automatically calculate the values of ICT, IRT, ET, and MPI. The values measured by this method are similar to those reported by Ghawi et al.  and Hamela-Olkowska et al. .
The present study unveiled that there was no significant difference in normal reference range of RMPI between IVF fetuses and SP fetuses. This may suggest that there was no obvious cardiac abnormality in IVF fetuses during the fetal period, or the change of cardiac function may not occur at the beginning of the IVF fetus. The normal reference range of cardiac RMPI in the SP fetus and the IVF fetus can be shared as well.
Valenzuela-Alcaraz et al. pointed out that the cardiac and vascular remodeling exists in the fetuses received by ART . In theory, compared with IVF fetuses, ICSI fetuses were not fertilized by natural selection. During the operation of ICSI, there is a greater threat to children’s health. Compared with IVF, ICSI bypasses the natural selection barrier of oocytes to sperm and may inject poor quality sperm to pass on certain genetic defects and high genetic risk genes to the next generation [12-13]. This may indicate that ART technology may affect fetal cardiac function, while IVF may be relatively safe. However, we still cannot conclude that IVF has no influence on fetal cardiac function.
There were a number of limitations in the present study. Firstly, the sample size of IVF fetuses was extremely small. Secondly, we did not collect ICSI fetuses for the study. Collecting ICSI fetuses for making comparison with other parameters may increase the accuracy of our findings. In addition, this was a cross-sectional study and was limited to the second and third trimesters of pregnancy . Last but not the least, our study lacked an invasive gold standard to determine the accuracy of RMPI measurements. Thus, further experiments with large sample size are required to confirm our findings and eliminate the above-mentioned deficiencies.