Diameter of the normal fetal thymus on ultrasound

Objective To present the normative data of the transverse diameter of the developing fetal thymus. Methods In this prospective study, the maximum transverse diameter of the thymus was measured by one sonologist in 376 normal fetuses between 19 and 38 weeks of gestation. We assessed the relationship of the transverse thymic diameter with gestational age (GA), biparietal diameter, femur length and abdominal circumference using general linear regression modeling. The predicted mean and 95% reference range of thymic diameter at each GA were calculated from the regression equation. Results Measurements of the transverse diameter were possible in 352 of the 376 (94%) fetuses. The transverse diameter of the fetal thymus increased with increasing GA and fetal size parameters in a linear manner. The regression equation for transverse diameter of the thymus as a function of GA was: thymic diameter (cm) = 0.15 × GA (weeks) − 1.59 (r2 = 0.86, P < 0.001). Conclusion The transverse diameter of the fetal thymus is easy to measure; this study presents normative data. Copyright © 2007 ISUOG. Published by John Wiley & Sons, Ltd.