Objective. To quantify the normal distance between the insertion of the medial leaflets of the mitral valve and tricuspid valve in the fetal heart. This mitral valve-tricuspid valve distance was compared with the distance from known cases of Ebstein anomaly. Methods. An apical 4-chamber view was obtained at end diastole in fetuses between 18 and 41 weeks' gestation. Calipers were placed parallel to the ventricular septum, with 1 caliper on the medial insertion of the mitral valve and a second caliper on the medial insertion of the tricuspid valve. The distance recorded was plotted against gestational age. Statistical analysis was performed by descriptive and linear regression techniques. Results. One hundred forty-five fetuses were studied. The mean ± SD mitral valve-tricuspid valve distance in the second trimester was 2.8 ± 0.9 mm with a range of 1.2 to 5.0 mm; in the third trimester it was 4.6 ± 1.1 mm with a range of 2.2 to 6.9 mm. Regression analysis showed that with each 1-week increase in gestational age, there was an increase of 0.15 mm in separation between the medial leaflets of the mitral valve and tricuspid valve (β = 0.15 ± 0.011). Conclusions. A positive correlation between mitral valve-tricuspid valve distance and advancing gestational age was found. The reference range described allows for the identification of a fetal heart with normal variation in the mitral valve-tricuspid valve distance. Further downward displacement of the medial tricuspid cusp suggests the possibility of Ebstein anomaly. Key words: Ebstein anomaly; fetal echocardiography; tricuspid valve. he prenatal evaluation of fetal anatomic structures includes a 4-chamber view of the fetal heart. In the normal fetal heart, the tricuspid valve and mitral valve are offset; the tricuspid valve has an apical displacement in relation to the mitral valve. The relationship of the atrioventricular (AV) valves in the fetal heart is evaluated in a subjective manner. Occasionally, there can be difficulty in determining whether the observed displacement is within a normal range. In Ebstein anomaly, the medial leaflet and sometimes the posterior leaflet of the tricuspid valve are located closer to the apex of the heart than normal. [1][2][3][4] Although this cardiac anomaly is rare, it can lead to heart failure in the antenatal period and can be difficult to repair. [5][6][7][8] Prenatal suggestion of this anomaly can facilitate a more detailed evaluation in the prenatal and neonatal periods and can allow for the education of parents to the potential implications of Ebstein anomaly.