Objective: To quantify the correlation between gestational age and the normal distance between the insertion of the medial leaflets of the mitral valve and tricuspid valve in Thai fetuses.
Material and Method: A prospective study was constructed using an apical 4-chamber view at end diastole to measure the mitral valve-tricuspid valve distance (MTD) in 169 confirmed-dated singleton normal fetuses between 18 and 40 weeks of gestation at King Chulalongkorn Memorial Hospital from August 2005 to July 2006. The recorded distances were plotted against gestational ages. The 5th, 50th, and 95th centile of MTD were calculated and the relationship between MTD and gestational age was determined with regression model. One hundred and forty-seven measurements were validated for analyses.
Results: Regression analysis demonstrated a linear correlation of MTD with gestational age with each increase of 1 week in gestational age, there was an increase of 0.12 mm of MTD (p <.001). The linear regression curve that correlated MTD with gestational age showed a gradual slope (r = 0.84; p < 0.001). The MTD range from 1.3 to 5.1 mm (mean + SD = 2.6+ 0.9). A significant difference in MTD was noted on comparing to the data in Western study. Intra-observer reliability was 0.92.
Conclusion: The present findings support the positive correlation between MTD and advancing gestational age. However, clinically significant differences of MTD between the presented population and Caucasian fetuses underline the need for employing a locally derived data on assigning the fetal cardiac abnormality.

Keywords: Ebstein’s anomaly, Echocardiography, Endocardial cushion defects, Tricuspid valve insufficiency, Congenital heart defect