Assessment of the intraobserver variability in the measurement of fetal cardiothoracic ratio using ellipse and diameter methods

OBJECTIVE: To assess the variability of fetal heart and thoracic area and circumference measurements using the ellipse and diameter methods at different gestational ages. DESIGN: This was a prospective cross-sectional study of 200 singleton pregnancies, with no apparent fetal abnormalities. The gestational age ranged between 19 and 42 weeks. At each examination, two transverse frames of the fetal chest at the level of the four-chamber view during diastole were obtained. For each frame, the area and the circumference of the heart and thorax were obtained by the ellipse and diameter methods. In the diameter method, cardiac and thoracic areas and circumferences were derived from the measured anteroposterior and transverse diameters. All measurements were made by the same observer. RESULTS: For the same method, intraobserver variability was good. Coefficient of variation for all measurements varied between 3-11%. The ellipse method tended to give larger measurements than did the diameter method, the absolute difference increasing with fetal age. The mean ellipse/diameter ratio was 1.10, 1.04, 1.05 and 1.02 for heart area, thorax area, heart circumference and thorax circumference, respectively. In most cases, intraobserver variability of the thorax measurements was independent of gestational age. In contrast, there was a greater variability of heart measurements with increasing gestational age. CONCLUSIONS: Both the ellipse and diameter methods of measuring cardiothoracic ratio are clinically useful and fairly reproducible at certain gestations, but are not interchangeable as the ellipse method overestimates measurements compared to the diameter method. The less reproducible measurement of heart size in later gestation should be taken into account when assessing cardiothoracic ratios.