Other causes associated with the onset of the syndrome in the case studies included multiple pregnancies (10 cases); viral infections such as Coxsackie B, cytomegalovirus, and parvovirus B19 (nine cases); tumors and congenital anomalies such as placental chorioangioma, fetal aortic stenosis, Galen's vein
aneurysm, cystic hygroma in the fetal neck, and fetal sacrococcygeal teratoma, among other unknown causes.
Monitoring is indicated in cases of fetal arrhythmia, nonimmune hydrops, placental chorangioma, and aneurysm of Galen's vein
. Level II and III evidence support the use of M-mode echocardiography to diagnose arrhythmias, and continuous fetal heart rate monitoring to determine the amount of time spent in sinus rhythm.