For oral chronic prophylaxis, drugs such as metoprolol, propranolol, and digoxin are considered safe first-line agents because of their longer record of safety, yet caution is advised, given that therapy with beta blockers has been associated with intrauterine growth retardation.517,524 This effect appears to be especially pronounced with atenolol, especially in mothers who received atenolol earlier in gestational age and who were treated for longer duration.525 Flecainide and propafenone have been used effectively to treat a variety of maternal and fetal tachycardias, yet they are reserved for patients who have no underlying structural heart disease or ischemic heart disease.113 IIb C-LD Catheter ablation may be reasonable in pregnant patients with highly symptomatic, recurrent, drug-refractory SVT with efforts toward minimizing radiation exposure.526,527 See Online Data Supplement 20. The risk of radiation exposure to the fetus is a concern with catheter ablation in pregnant patients, because high-dose ionizing radiation has been linked to excess malignancy and congenital malformations.528 However, the fetal radiation dose for most common cardiovascular interventions is not likely to exceed the 50-mGy negligible-risk threshold dose for excess malignancy.529 One study that used phantoms to simulate pregnancy estimated a low lifetime risk of malignancies from radiation exposure to the conceptus during a typical ablation procedure.527 Furthermore, with current technologies such as electroanatomic mapping systems, catheter ablation procedures using minimal or even zero fluoroscopy have been described in pregnant women.526 Thus, if a catheter ablation procedure is required in a pregnant woman, radiation-reduction technologies should be used, and the procedure should be avoided in the first trimester when the teratogenic risk is greatest.528 Of note, shielding the fetus by covering the mother with a lead apron does not eliminate radiation to the fetus because most of the radiation to the fetus comes from scatter. 