Sonographic measurement of fetal thymus size in uncomplicated singleton pregnancies

Purpose: To establish sonographic reference ranges of the normal fetal thymus size between 17 and 38 weeks of gestational age (GA). Methods: The study was conducted between April 1 and December 31, 2013. Low-risk singleton pregnancies without obstetrical and medical complications at the GAs between 17 and 38 weeks were recruited for thymus measurement. The fetal thymus was identified on transabdominal sonography at the three-vessel view. Maximal transverse diameter, perimeter, and thymus/thoracic ratio were measured. The best-fit models in predicting thymic dimensions as a function of GA and biparietal diameter (BPD) were determined using regression analysis, and percentile charts for predicting thymic dimensions were constructed. Results: A total of 296 singleton pregnancies were recruited in this study. Maximal transverse diameter, perimeter, and thymus/thoracic ratio increased throughout pregnancy. The regression equation for maximal transverse diameter of the thymus as a function of GA was as follows: Predicted mean thymus diameter (mm) = −25.904 + 2.476 × GA − 0.019 × GA² (r = 0.915; p < 0.001) with predicted standard deviations of thymus diameter (mm) = 1.428 + 0.044 × GA (r = 0.017; p < 0.001). Conclusions: Sonographic reference ranges of the normal fetal thymic dimensions between 17 and 38 weeks of GA have been established.