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To establish the reference ranges of the fetal thymus size among Thai fetuses.The database of spatio-temporal image correlation (cardio-STIC) was assessed to obtain the volume data sets for offline analysis. The volume data sets acquired at 16 to 38 weeks were measured for the thymus transverse diameter (TD) and the thymus/thoracic (TT) ratio at the three-vessel view. The measured values were regressed to identify the best-fitted model.A total of 622 volumes were successfully measured and the reference ranges of TD and TT ratio were established. Although TT was relatively constant or increased minimally with gestational age (GA), TD was significantly increased with gestational age. The predicted mean TD (mm) = −31.206 + 2.854 × GA − 0.028 × GA2 (r = 0.891; P < .001) and predicted standard deviation (SD) of TD (mm) = 0.837 + 0.073 × GA (r = 0.038; P < .001). Centile charts for predicting TD and equations for z-score calculation were also provided. Interobserver variability in TD measurement was better than that in TT ratio and thymic perimeter measurement.Reference ranges of fetal TD and TT ratio have been provided. These normative data may be a useful tool in the assessment of thymus-associated conditions. For clinical purposes, we recommend measurements of the TD rather than TT ratio or perimeter because of its reproducibility and simplicity.