First-trimester fetal heart block: a marker for cardiac anomaly

Fetal heart block in the second and third trimesters may be caused by transplacental passage of auto-antibodies or cardiac defects. Little is known about the etiology of first-trimester fetal heart block.

Fetal heart block was diagnosed in four patients (negative antibody serology) referred for first-trimester sonographic evaluation of increased fetal nuchal fold thickness with bradycardia. Two-dimensional echocardiography was complemented by color Doppler flow imaging of the fetal heart. Fetal arrhythmia was verified by M-mode, pulsed wave Doppler and/or color M-mode echocardiography.

All fetuses had congenital heart disease, atrioventricular valve regurgitation, heart block and edema. Autopsy confirmed heterotaxy in three cases (left atrial isomerism with atrial septal defect; left isomerism with double-outlet right ventricle, great artery malposition and ventricular septal defect. The third case had dextrocardia with atrioventricular canal defect and the fourth case dextrocardia with great artery transposition.

First-trimester fetal bradycardia may result from heart block of the type associated with complex congenital heart disease. Accelerated edema formation in this setting may be the basis of nuchal edema formation. First-trimester fetal echocardiography offers the potential for early diagnosis and intervention in these cases with poor prognosis.