Gestational Age-Specific Reference Ranges and Internally Validated Z-Score Equations for Fetal Thymus Transverse Diameter at 18 + 0–25 + 6 Weeks of Gestation

Background/Objectives: The fetal thymus is increasingly assessed as a marker of fetal wellbeing, yet reliable gestational-age-specific reference data remain limited. We aimed to establish reference ranges, centile charts, and internally validated Z-score equations for the thymus transverse diameter at 18 + 0–25 + 6 weeks of gestation. Methods: This retrospective cross-sectional study was conducted at a single tertiary fetal medicine centre between January 2023 and December 2025. Structurally normal singleton pregnancies with a reliable gestational age were eligible. The maximum transverse diameter of the fetal thymus was measured in the three-vessel trachea view. The mean was modelled by polynomial regression and, after heteroscedasticity was confirmed, a gestational-age-specific standard deviation was modelled using the Royston–Wright approach. Z-score equations and smoothed centiles were derived, and performance was assessed by bootstrap, cross-validation, and split-sample validation. Results: A total of 2103 pregnancies were included. A cubic mean model was selected, and variance increased significantly with gestation (Breusch–Pagan p = 1.34 × 10−18). The predicted median increased from 9.7 mm at 18 weeks to 18.3 mm at 25 weeks. Fetal sex was statistically associated with thymus transverse diameter, but the effect was negligible (adjusted mean difference 0.247 mm; p = 0.011; Cohen’s d = 0.051; interaction p = 0.949), so a single reference was retained. Excluding maternal comorbidities changed the median by at most 0.24 mm. Bootstrap optimism was 0.006 mm and the 10-fold cross-validated root-mean-square error 1.863 mm. Conclusions: We provide contemporary gestational-age-specific reference ranges and internally validated Z-score equations for the thymus transverse diameter between 18 + 0 and 25 + 6 weeks, which may support clinical assessment and future research, pending external validation.