Objectives: Echocardiography is the most commonly used modality for diagnosing fetal arrhythmias. Although simultaneous Doppler recording at the superior vena-cava (SVC) and the ascending aorta (aAo) is thought to be the most reliable method for analyzing the relation between the atrial and ventricular contraction, the relatively difficult technic and unclear interpretation are often problem for this method. In this study, we evaluated the relation between a small diastolic flow in the aAo (SDFAo) and an atrial contraction, and assessed the usefulness of making analyzing fetal heart rhythms using the SDFAo. Methods: First, to find the cause of the SDFAo, we obtained simultaneous recording of electrocardiogram, Doppler echocardiographic wave form at the aAo, color Doppler mapping, and color M-mode in neonates. Next, we measured AV interval in 83 fetal echocardiographic examinations using the two methods, the simultaneous SVC-aAo Doppler waveform and the SDFAo, and compared. Finally, we assessed fetal arrhythmias in SDFAo method and assessed the usefulness for assessing the heart rhythms. Results: In the neonatal examination, SDFAo is found to be created by posterior movement of the aAo due to atrial contraction. The interval between SDFAo and systolic flow at the aAo is the same as the PR interval by ECG. Next, in the fetal examination, AV interval measured using the SDFAo method is well correlated to that with the SVC-aAo method (r2 = 0.322, P < 0.001). Finally, in the case with fetal heart block, the SDFAo method was much easier and clearer for assessing atrial contraction than the SVC-aAo method. However, the SDFAo method can not detect atrial contraction during fetal tachycardia and paroxysmal contraction. Conclusions: The SDFAo method can be used to detect atrial contraction for measuring AV interval, and for analyzing fetal bradyarrhythmias. This method is much easier and more practical for analyzing fetal heart rhythms than SVC-aAo method.