Fetal Atrioventricular Block and Postpartum Augmentative QT Prolongation in a Patient With Long‐QT Syndrome With KCNQ1 Mutation

2:1 AV Block in KCNQ1. The case of a 32‐year‐old pregnant woman, who had had several syncopal episodes during swimming and running at 9 and 10 years of age and whose fetus had 2:1 AV block, is presented. The mother and baby had the same heterozygous single nucleotide substitution in KCNQ1 at T587M. After 27 weeks of gestation, the fetal 2:1 AV block disappeared, and 1:1 AV conduction resumed, with a fetal heart rate of 110–120 beats/min. The maternal electrocardiogram revealed a normal QTc interval (433 ms) without ST‐T abnormalities at gestational week 23, but the QTc was 490 and 531 ms at 1 and 2 months postpartum, with biphasic T waves in leads V2 and V3. This case is the first report of fetal 2:1 AV block with KCNQ1 mutation (T587M) and unmasked maternal QT prolongation in the postpartum period. (J Cardiovasc Electrophysiol, Vol. 21, pp. 1170‐1173)