Background: Magnetocardiography and M-mode fetal echocardiography are non-invasive techniques capable of identifying fetal arrhythmias. The STAN®-fetal scalp electrode system can record the fetal echocardiogram in labor. Case: A patient was admitted to hospital with preterm contractions and cervical insufficiency at 28 weeks of gestation. After treatment with a β-sympathomimetic drug (Partusisten®) one fetus developed supraventricular tachycardia. After terminating the Partusisten medication, there was no effect on the fetal arrhythmia and flecainide therapy was initiated. Maintenance dosages controlled the condition thereafter. Cardiac time intervals of a fetus in labor can be presented, which did not change significantly throughout the first stage of labor. Conclusion: Flecainide is an effective therapy for supraventricular tachycardias in a twin pregancy. Analyzing the cardiac time intervals during pregnancy can improve perinatal outcome.