['Fetal tachyarrhythmia can lead to fetal hydrops due to heart failure. Flecainide is often considered as second-line therapy when digoxin monotherapy fails, which is more likely in hydropic fetuses. Time to conversion to sinus rhythm (SR) is critical in cases presenting with hydrops.', 'The aim of this study was to evaluate the efficacy and time to conversion to SR of transplacental treatment, especially flecainide.', 'This is a retrospective observational study of 46 fetuses with fetal tachyarrhythmia. Treatment was either flecainide (n = 28, 60.9%), digoxin+flecainide combination (n = 4, 8.7%), or digoxin (n = 10, 21.7%). In 4 fetuses (8.7%), no treatment was necessary.', 'In our study population, 26 of the 32 fetuses (81.2%) that were treated with flecainide as a first-line therapy (flecainide or digoxin+flecainide) converted to SR. The median time to conversion to SR was 3 days (range 1-7 days) with flecainide monotherapy and 11.5 days (range 3-14 days) with a combination therapy. Seventy-two percent (13/18) of hydropic fetuses and 90% (9/10) of nonhydropic fetuses converted to SR when treated with flecainide monotherapy. There was no statistical difference in rates of conversion to SR in hydropic and nonhydropic fetuses (P = .37) or time to conversion to SR in the 2 groups (P = .9). In the majority of the remaining fetuses, there was a partial response with decreased ventricular heart rates that were well tolerated.'] Is high-dose flecainide the most effective treatment of fetal supraventricular tachycardia?