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Objective: In clinical practice, measurement of mechanical PR interval (mPR) with pulsed Doppler echocardiography is a standard method used to estimate the atrioventricular conduction time in the fetus. However, fetal echocardiography does not directly reflect the electrical properties of the heart. Technological advances in fetal magnetocardiography (fMCG) have allowed recording of the electrical PR interval (ePR) with high time resolution. The aim of this study was to clarify the differences between ePR and mPR. Methods: The study subjects were 295 normal human fetuses (gestational age, range 20.4-41.4 weeks) who underwent fMCG, and 135 of them underwent fetal echocardiography 15-90 min before or after fMCG. The ePR was measured using the fMCG, and the mPR was determined by two pulsed Doppler methods, simultaneous recording of the left ventricular inward and outward flow (LV in/out) (n = 135) and superior vena cava and ascending aorta (SVC/aAo) (n = 84). Results: The ePR showed a significant, but weak, positive correlation with gestational age (r = 0.162, p = 0.0053). The mPR was significantly longer than the ePR (p < 0.0001), with mean differences of 14.6% (95% limits of agreement -10.7, 39.9) for the LV in/out method and 14.7% (95% limits of agreement -8.6, 38.0) for the SVC/aAo method. Conclusion: Our results point to the risk of overestimation of the atrioventricular conduction time when the mPR is used, and the need for careful interpretation of PR prolongation determined by mPR. Copyright © 2012 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]