The peripheral hypoechoic rim of the fetal heart

The purpose of this study is to determine the origin of the hypoechoic rim around the fetal heart; this rim has been noted previously but there is disagreement as to its origin. Of 314 fetuses scanned, a four-chamber view was obtained in 290; a hypoechoic rim was present in 94% of these. In all cases in which a short-axis view could be obtained, the rim continued circumferentially through the ventricular septum. Because the rim continues through the septum, it cannot be pericardial fluid but is instead part of the myocardium. The presence as well as the origin of this normal hypoechoic rim around the fetal cardiac ventricles should be considered before diagnosing small pericardial effusions.