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Background Congenital long QT syndrome (LQTS) type 1 is characterized by abnormally prolonged ventricular repolarization caused by inherited defects in cardiac potassium channels. Patients are predisposed to ventricular arrhythmias and even sudden cardiac death. In some cases, foetal sinus bradycardia is the only sign, making prenatal diagnosis challenging. Physicians should be aware of this subtle presentation of LQTS. Early diagnosis and proactive treatment are crucial for preventing unexpected cardiac events. Case summary A healthy and asymptomatic 25-year-old pregnant woman was referred to our institute for cardiac evaluation after persistent foetal sinus bradycardia was detected during repeated ultrasounds, despite the absence of any foetal morphological or functional cardiac anomalies. After a thorough assessment, the mother was diagnosed with LQTS type 1, as confirmed by molecular genetic testing. Appropriate management, including maternal medication and increased surveillance, was initiated. The infant was delivered safely, and his electrocardiogram revealed a significantly prolonged QTc interval. Genetic testing confirmed the maternally inherited variant in KCNQ1 gene, and beta-blocker therapy was started. No arrhythmic events were noted. Discussion Detection and careful stratification of foetal heart rate (FHR) is crucial in every pregnancy. Foetal bradycardia can be caused by both maternal and foetal factors. Persistent low FHR should raise a high suspicion for LQTS. The condition may also present with atrioventricular blocks, torsades de pointes, or sudden intrauterine foetal demise. Accurate and early diagnosis of LQTS is essential for implementing appropriate management strategies, which include vigilant monitoring, effective medical treatment, careful planning of delivery, and post-natal care. [ABSTRACT FROM AUTHOR]