Fetal thymus size in uncomplicated twin and singleton pregnancies

The main objective of this study was to determine whether fetal thymic measurements could be obtained in twins, with a secondary goal to determine whether thymic measurements from uncomplicated singleton and twin pregnancies are comparable.

The transverse diameter and perimeter of the fetal thymus were measured prospectively in 678 singleton and 56 twin pregnancies, and their relationships with gestational age were determined and compared between groups.

Thymic measurements were possible in 757 (95.8%) of the 790 fetuses. Measurements were not possible in 19 of 678 singletons (2.8%) and in 14 of the 112 (12.5%) twins (P < 0.001). After construction of nomograms for the transverse diameter and perimeter of the fetal thymus, similar measurements were noted for singletons and twins.

These results suggest that sonographic measurements of the thymus are feasible in twin pregnancies and that, in uncomplicated pregnancies, these measurements are similar to those noted for singletons. These findings pave the way for future studies aimed at determining the clinical utility of thymic measurements in complicated singleton and twin pregnancies.