Reference centiles for fetal thymus transverse diameter at 18-25 weeks of gestation: a cross-sectional ultrasound study

Abstract

Aims: To establish gestational age-specific reference centiles for fetal thymus transverse diameter between 18 and 25 weeks of gestation, corresponding to the routine mid-trimester anatomical survey.Methods: This retrospective cross-sectional study included singleton pregnancies that underwent detailed second-trimester ultrasonography between May 2023 and March 2025. Fetal thymus images were retrieved from the institutional Picture Archiving and Communication System (PACS). Cases with structural or chromosomal anomalies, maternal systemic disease, multiple gestations, incomplete thymus imaging, or absent postnatal follow-up were excluded. The transverse diameter of the fetal thymus was measured in the three-vessel and trachea view by two maternal-fetal medicine specialists. Intraclass correlation coefficient (ICC) analysis was performed to assess interobserver agreement. The relationship between gestational age and thymus size was evaluated using Pearson correlation and linear regression. Gestational age-specific centiles were constructed.Results: A total of 407 fetuses between 18 and 25 weeks of gestation were included. Fetal thymus transverse diameter demonstrated a significant positive correlation with gestational age (r=0.567, p