The aim of this paper is to describe impacts of fetal magnetocardiogram on clinical managements of fetal cardiac diseases. We used a system with 64-channel SQUID sensors. Fetal magnetocardiograms were especially useful for the diagnosis of fetal Wolff-Parkinson-White syndrome and fetal QT prolongation by measuring PR, QRS and QT intervals in magnetocardiogram. These findings were of benefit to selecting drugs of intrauterine therapy. Long-duration, 20 minutes, recording of magnetocardiogram was almost stable except for the duration with large fetal movements and was useful for denying paroxysmal attacks of life-threatening arrhythmia. However, there were a few minor problems such as maternal back pains and supine hypotension syndrome due to the nature of pregnant woman. Avoiding maternal back pains and supine hypotension syndrome, and decreasing fetal movements may be needed for the better use of long-duration magnetocardiogram.