OBJECTIVE:To determine if the incidence of maternal diabetes mellitus or neonatal macrosomia is more frequent in fetuses and neonates with atrial arrhythmias than the general population. METHODS:Fetuses and neonates <30 days of age with atrial flutter or ectopic atrial tachycardia and structurally normal hearts were identified retrospectively through the cardiology databases. Electrocardiograms, echocardiograms, and medical records of mothers and infants were reviewed. RESULTS:Thirty-one patients (15 fetuses, 12 diagnosed in-utero) were identified. Infants with atrial flutter or ectopic atrial tachycardia were more likely to be macrosomic or to be born to diabetic mothers than the general population. Two had left atrial dimension z-scores above +2, and two had interventricular thickness z-scores above +2. Eighteen of 19 had abnormal mitral E/A ratios, suggesting left ventricular diastolic dysfunction. CONCLUSIONS:Fetuses and neonates with atrial flutter or ectopic atrial tachycardia were more likely to be macrosomic or be born to diabetic mothers than the general population. Postnatal echocardiography suggests that there may be abnormal diastolic left ventricular filling in some babies with these arrhythmias. Independent of ventricular hypertrophy, we speculate that isolated, non-recurrent fetal or neonatal atrial flutter, or ectopic atrial tachycardia may be caused by cardiac diastolic dysfunction and atrial stretch in utero.