INTRODUCTION: Inappropriate sinus tachycardia (IST) is rare in the general population and its etiology is unknown. First described by Bauernfeind et al, it is characterized by a persistently high heart rate (HR) and an exaggerated response to physical activities. It has non-defined mechanisms. The IST is suggested to be a primary abnormality of the sinus node. Tachyarrhythmias in fetal life may cause heart failure and intrauterine death. OBJECTIVES: Report an IST case in a seven-week gestation fetus emphasizing the importance of fetus echocardiography in early phase of gestation. METHODS: Pregnant patient referred due to persistent fetal tachycardia following obstetric ultrasonography. The fetus presented an initial frequency of around 240 beats/min (bpm) with 1:1 atrioventricular conduction with no hydropsy in the transvaginal echocardiogram. It was about an IST, being the differentiated supraventricular tachycardia diagnostic initially difficult. A digital therapy with the mother was started with no success. Flecainide was used next, again to no avail. Solatol oral use was then chosen, there initially being a reduction of fetal myocardium HR to tolerable levels (180 bpm). The number of heartbeats normalized at thirty six weeks gestation (140 bpm). It showed a persistent high basal HR in a reassessment after the neonatal period (180 bpm), increasing excessively at minimum efforts and P wave in sinus rhythm morphology on ECG. Possible sinus tachycardia causes were dismissed, thus confirming the above diagnostic. CONCLUSION: The authors report the importance of fetal echocardiography for the diagnosis and early therapy of fetal arrhythmias to avoid complications.