Fetal arrhythmia was investigated in 148 fetuses. Eight of these fetuses displayed signs of redundancy/aneurysm of the foramen ovale, all in combination with various atrial arrhythmias. At presentation 4 (50%) of these fetuses had mild to moderate congestive cardiac failure and a 5th fetus developed mild cardiac failure four weeks after observation of the aneurysm. All fetuses survived, but 2 fetuses required early delivery because of the cardiac failure. Unnecessary morbidity resulting from the possible development of cardiac failure may be prevented by regular monitoring in these cases.