In the fetus, the Eustachian valve or valve of the inferior vena cava directs the flow of oxygen-rich blood through the right atrium and across the foramen ovale into the left atrium. There is wide variability in the size, shape, and thickness of the persistent Eustachian valve: It may almost completely regress, with only a subtle, thin ridge persisting, or it may persist as a mobile, elongated fold at the junction of the inferior vena cava and the right atrium. Laterally, the Eustachian valve meets the crista terminalis; medially, it joins a fold at the orifice of the coronary sinus (thebesian valve). The mobility of the residual valve may mimic thrombus or, rarely, a mass (Figs. 14.3 and 14.4).