Nomogram of Fetal Thymus Using Thy-Box Technique in South Indian Population

To create a nomogram of fetal thymus using the thy-box technique from a single center in South Indian population. This is a retrospective study that included 723 singleton pregnancies referred for anomaly scan. The study was conducted at a Fetal Medicine center in South India. The thymus was localized in the axial plane of the upper mediastinum of the fetus. At first, the internal mammary arteries that course laterally to the thymus were located by using color or power Doppler ultrasonography with a low pulse repetition frequency of these vessels. This was used to identify its lateral borders as described by Paladini et al. (Ultrasound Obstet Gynecol 37(4):488–492, 2011). The thy-box, was thereby readily displayed in the fetus. The anteroposterior and transverse diameters of the thymus using thy-box were measured and recorded. The fetal thymus was visualized using the thy-box technique in a total of 723 cases. Thymus transverse diameter as a function of gestational age was expressed by the regression equation: Transverse diameter (in mm) = 0.08115 × Gestational age − 0.5366 [R square = 0.98 and p < 0.0001]. Similarly the anteroposterior diameter (AP) as a function of gestational age was expressed by the equation: AP (in mm) = 0.0449 × Gestational age − 0.2759 [R square2 = 0.9686 and p < 0.0001]. The mean, standard deviation and 95% confidence interval for the thymus measurements corresponding to each gestation age were also calculated and tabulated. Linear regression model was used and data was analyzed using IBM SPSS version 22 software (Armonk, NY). The fetal thymus can be quickly measured with reliability and reproducibility in fetuses during the anomaly scan, when the thy-box technique is employed. There is no significant ethnic variation in thymus size. Moreover, an evaluation of the fetal thymus if added as a routine parameter during the performance of the second trimester anomaly scans, would enhance the ability to identify thymic hypoplasia when present.