Inappropriate sinus tachycardia (IST) is characterized by an inexplicably elevated sinus heart rate (HR) at rest, with an exaggerated response to physical activity. Its etiology is unknown, and IST is more common in the middle-aged population, being rare in children and adolescents. Although a favorable long-term prognosis has been described in a small series, tachycardia-induced cardiomyopathy has rarely been associated with IST. In this report, we describe an extremely rare case of IST detected in the first trimester of gestation, focusing on reviewing important features to improve the diagnosis of the different types of tachycardia by applying fetal cardiac ultrasonography. In the case reported, the diagnosis of IST was suspected in a fetus from a pregnant woman with celiac disease, who was referred at 9 weeks of gestation with fetal tachycardia. The fetus remained hemodynamically stable until term, and the female neonate was born unremarkably. Electrocardiography in the newborn showed sinus tachycardia (ST) with normal PR and QTc intervals. Possible causes of ST were excluded, thus confirming the fetal diagnosis. The patient presented with a fast HR at rest with an exaggerated increase with minimal physical activity accompanied by irritability. Beta-adrenergic blocker therapy was initiated, and the female neonate has progressed well.