Abstract
<title>Abstract</title> <p>Objective. Ultrasonographic assessment of the fetal thymus can be performed with different techniques whose results are sometimes used interchangeably. The aim of this study was to compare two thymus measurement methods - a technique based on a standardized acoustic window (THY-box) and direct two-dimensional (2D) measurement - in terms of feasibility across consecutive trimesters and agreement between the methods, and to construct method-dependent nomograms referenced to gestational age. Methods. A cross-sectional study was performed comprising 147 fetal thymus measurements obtained from 119 singleton pregnancies. Each measurement was assigned to a trimester on the basis of gestational age (first trimester below 14 weeks, second trimester 14 to 27 weeks, third trimester from 28 weeks), yielding 22, 91 and 32 measurements, respectively. In 26 fetuses both methods were applied within the same examination. Agreement was assessed using Bland-Altman analysis, Deming regression and the intraclass correlation coefficient (ICC). Nomograms were constructed by second-order polynomial regression on independent observations (one measurement per pregnancy), with the standard deviation modelled as a function of gestational age. p values below 0.05 were considered statistically significant. Results. In the first trimester measurement was feasible only with the THY-box technique; the isoechogenicity of the thymus and lung tissue precluded its delineation on 2D imaging. In the subgroup with paired measurement, 2D yielded systematically larger values than THY-box (mean difference 3.91 mm; limits of agreement − 1.50 to 9.31 mm), with a very strong linear correlation (r = 0.931; p < 0.001) and no significant proportional trend (p = 0.924). The ICC was 0.82. Both methods showed a strong relationship between thymus dimension and gestational age (THY-box: rho = 0.836; 2D: rho = 0.664; p < 0.001), with a smaller residual scatter for the THY-box nomogram. Gestational-age-adjusted thymus dimension was positively associated with fetal weight percentile (rho = 0.404; p = 0.001) but not with nuchal translucency, uterine artery pulsatility index, fetal sex, mode of conception or recent infection. Conclusions. The THY-box and 2D techniques measure the same structure with excellent correlation but differ by a nearly constant component of about 4 mm, which renders them non-interchangeable. Reference ranges developed for one method should not be applied to measurements obtained with the other. Method- and trimester-dependent nomograms organize the assessment of the fetal thymus across successive stages of pregnancy.</p>