We report the case of a fetal supraventricular tachycardia with hydrops treated by transplacental administration of digoxin and flecainide. The maternal serum concentration of digoxin and flecainide fluctuated throughout the third trimester. Therefore, monitoring the maternal concentrations of these drugs may improve efficacy and tolerability. | WHAT IS KNOWN AND OBJECTIVEFetal tachyarrhythmia occurs in less than 0.1% of pregnancies. The most common type of fetal tachycardia is supraventricular tachycardia (SVT), which may cause fetal hydrops. 1 Fetal hydrops is associated with a mortality rate as high as 35% compared with 0%-4% in non-hydropic fetuses. 2