The fetal thymus may be visualized using ultrasonography (USG) and is typically located in the mediastinum. In the past years, the size of the fetal thymus has served not only as a marker of genetic or heart defects but also as a predictive factor for intrauterine growth restriction, premature birth, preeclampsia, chorioamnionitis or even neonatal sepsis.

Material and methods:
A total of 410 fetuses were qualified for the study. Fetuses with heart defects were excluded from the study. The fetal thymus was evaluated with ultrasonography between the 14th and 40th week of gestation. After obtaining a standard transverse view encompassing the three great vessels, thymus measurements were attempted, i.e. maximal transverse diameter, circumference and surface area. Linear regression was used for statistical analysis, yielding 3 models, each with a different dependent variable. The confidence interval for each model was set at 80% to aid the comparison with centile grid growth charts for neonates and children. The test was regarded as statistically significant when p < 0.05.

From a total of 410 fetuses the thymus transverse diameter, circumference and area were successfully measured in 410, 320 and 330 cases, respectively. The probabilities are lower than 0.0005 for each model, which means that each model is quite statistically significant.

The coverage of healthy thymus nomograms in the fetal population may be the basis for the identification of fetuses at risk of hypoplasia or thymic hyperplasia, which seems particularly important from the point of view of the detection of potential inborn immunological disorders.