In a series of over 7,000 pregnancies referred for fetal echocardiography, atrioventricular septal defects were detected in 103 fetuses, (17% of 594 anomalies). A large proportion (42%) also had isomerism of the atrial appendages, abnormalities in arrangement and morphology of the thoracoabdominal organs, and a high incidence of associated cardiac malformations. When this was associated with abnormalities of rhythm, the fetus often showed signs of intrauterine congestive heart failure; hydrops, pleural effusion and ascites. Those with trisomic karyotypes (predominantly trisomy 21) were associated with usual atrial arrangement and concordant connections of the cardiac segments. Of the 103 fetuses, 53 were examined at autopsy. Extracardiac abnormalities were documented, and detailed morphologic examination and measurements of the heart were made. The ratios of inlet to outlet septal disproportion were not significantly different from those found in postnatal hearts. Again, as in postnatal life, those hearts with common atrioventricular orifice had a greater degree of ‘scooping’ of the ventricular septum compared to those with separate right and left atrioventricular valves. There were no significant differences in these findings in relation to the karyotype of the fetus. There was an increased incidence of ventricular dominance, and also of coarctation or hypoplasia of the aortic arch, in fetal hearts when compared to autopsied hearts obtained in postnatal life. In summary, the distinguishing morphological characteristics of atrioventricular septal defect are present from 16 weeks of gestation.