In utero sonographic appearance of the atrial septum primum and septum secundum.

We present our observation of the sonographic appearance of the septum primum and septum secundum. The septum primum, as the valve of the foramen ovale, has been previously described as a mobile, echogenic line or dot in the left atrium. We believe that these previous descriptions recognize only a small part of a more complex three-dimensional shape. We studied 14 ideal patients, none of whom were referred for suspected fetal cardiac disease. In 1 case the septum primum was seen as a flaplike structure and in 8 cases as a circular structure. In 5 cases it was seen as both flaplike and circular, depending on the scan plane selected. A three-dimensional description of the shape of the septum primum, based on the work of Crelin, is proposed, which explains these two appearances. A better understanding of this anatomical feature may prevent mistaking the septum primum for an abnormality and provide the basis for future advances in fetal echocardiography.