A series of 14 women who were given flecainide 300 to 400 mg per day to treat fetal supraventricular tachycardias complicated by fetal hydrops and ascites has been reported. The drug was given at a mean gestational age of 31 weeks, and was continued for up to five days. The mean umbilical cord to maternal plasma ratio was 0.80, with fetal concentrations ranging from 400 to 800 mcg per liter. One intrauterine death was observed three days after flecainide was begun, that may have been due to a proarrhythmic effect of flecainide. A second infant died later of sudden infant death syndrome. No adverse effects or problems were noted in the remaining 12 infants.

There have been several cases in which flecainide was used safely during human pregnancy. This drug has been used safely and successfully to treat refractory (to digoxin) fetal tachycardia. Average umbilical cord blood to maternal serum flecainide concentration ratios ranging from 0.63 to 0.97 have been reported. However, at least one case of neonatal cardiotoxicity related to maternal use of flecainide for fetal supraventricular tachycardia has been reported.

Flecainide has been assigned to pregnancy category C by the FDA. Animal studies have revealed teratogenicity and embryotoxicity in rabbits that were given doses 30 to 35 mg/kg/day. No data from controlled studies in human pregnancy are available. Flecainide should only be given during pregnancy when there are no alternatives and benefit outweighs risk.

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Flecainide is excreted into human milk. Adverse effects on the nursing infant are unlikely. Flecainide is considered compatible with breast-feeding by the American Academy of Pediatrics.

Flecainide concentration in human milk averages 2.5 times that found in maternal plasma. Some experts have calculated that, assuming an infant consumes 700 mL of milk every 24 hours and the maternal plasma concentration is 1 mcg/mL, a suckling infant would be exposed to about 3 mg of flecainide per day. The calculated resultant infant plasma flecainide level would be 62 ng per mL. This represents a negligible amount of drug to the nursing infant.

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