We report high precision assessment of fetal rhythm in utero in a case of i
solated congenital complete heart block using fetal magnetocardiography. Th
e recordings reveal a remarkably strong tendency for the atria and ventricl
es to synchronize, which is manifested by the continual presence of ventric
ulophasic sinus arrhythmia and frequent episodes of accrochage and isorhyth
mic AV dissociation.