The case of a gestant diagnosed fetal arrhythmia in the 35th week of gestation is presented. A fetal echocardiography indicates an aneurysm of the interauricular septum and a fetal supraventricular tachycardia. Also is obtained an ambulatory 24-hour Holter electrocardiograph of the gestant and fetus rhythm. The register makes evident a gestant supraventricular tachycardia and as well a fetal supraventricular tachycardia, occurring in both without clinical company. Following the birth, the tachycardia in the newborn is identified to be automatic and incessant, requiring digital for its control. In postpartum registers of the mother no significant arrhythmia of any type is observed. We conclude that the ambulatory monitoring with a Holter electrocardiograph could be a good method of diagnosis and follow-up of fetal and gestant arrhythmias.