Additional Study Information: Although supraventricular tachycardia (SVT), including atrial flutter (AF), are the most common causes of intended in-utero fetal therapy, none of the medication used to date has been evaluated for their effects on the mother and her baby in a randomized controlled clinical trial (RCT). In the absence of such evidence, there is no consensus for the optimal management. The Fetal Atrial Flutter and supraventricular Tachycardia (FAST) Therapy Trial is a prospective multi-center trial that addresses this knowledge gap to guide future fetal atrial flutter & SVT therapy to the best of care.