Fetal supraventricular tachyarrhythmias are very rarely associated with congenital heart diseases. Usually a circus movement (= reentry), often associated with a preexcitation, and occasionally an atrial ectopic focus are suggested as the pathophysiologic mechanisms of supraventricular tachyarrhythmias. In the reported case - atrial flutter, complete common atrioventricular canal with fetal trisomy 18 - the atrial flutter was probably provoked by the dilatation of the atria due to the regurgitation of both atrioventricular valves. This case indicates the need for a detailed echocardiographic examination (using two-dimensional, M-mode, and Doppler-echocardiography) in each case of fetal tachyarrhythmias in order to exclude associated cardiac defects, possibly in association with a chromosomal anomaly.