Assessment of the fetal thymus by two- and three-dimensional ultrasound during normal human gestation and in fetuses with congenital heart defects

Abstract

Objectives: Our objectives were to compare the size and volume of the developing fetal thymus obtained by two-dimensional ultrasound (2D-US) and three-dimensional ultrasound (3D-US), develop normative data for thymus volume (TV), and investigate TV in fetuses with congenital heart disease (CHD) and normal twin gestations.

Methods: We studied 321 fetuses (gestational age (GA): 17-39 weeks) including 238 normal singletons, 64 normal twins and 19 singleton fetuses with CHD. We used 2D-US to assess fetal thymus maximum transverse diameter (MTD), maximum transverse area (MTA), anteroposterior diameter (APD) and superoinferior diameter (SID). TV was obtained by 3D-US using virtual organ computer-aided analysis. Measurements were adjusted for estimated fetal weight where appropriate. Linear regression analysis, general linear models and Fisher's Z-transformation were used where appropriate. A nomogram of fetal TV based on singleton gestations was produced according to previously published methods.

Results: Ultrasound assessment of the fetal thymus was possible in 95.3% (306/321) of cases. Both 3D-US and 2D-US measurements were significantly correlated with GA (TV r = 0.989; MTA r = 0.918; MTD r = 0.884; APD r = 0.849; and SID r = 0.816; all P < 0.05). After Fisher's Z-transformation, the correlation between the TV and GA was significantly stronger than that between any individual 2D-US measurement and GA (P < 0.05). Normal twin fetuses had TVs similar to those of singletons adjusted for estimated fetal weight and GA (P = 0.85). TV adjusted for estimated fetal weight and GA was significantly lower in fetuses with CHD than in normal singletons (P < 0.05).

Conclusion: 2D-US and 3D-US are useful tools for evaluation of the size and volume of the human fetal thymus through gestation. Fetal TV by 3D-US seems to reflect normal development of the thymus in utero better than do 2D-US measurements. Lower TV should be expected in association with CHDs.