Comparative sonographic measurement of the fetal thymus size in singleton and twin pregnancies

The aim of this study was to compare the size of the thymus gland of healthy fetuses in twin and singleton pregnancies using the thymic-thoracic ratio (TTR). As a second objective, the TTR difference between monochorionic and dichorionic twin thymus size was examined. Finally, a possible correlation between thymus size and gestational age (GA) at birth in twin pregnancies was investigated.

The TTR was measured retrospectively using the plane of three-vessel view in prenatal ultrasound examinations. Images of 140 healthy twin fetuses (33 monochorionic and 107 dichorionic) between 19+0 and 33+2 weeks of gestation were utilized and compared to 248 healthy singleton fetuses.

A significant difference in thymus size could be observed between healthy fetuses from singleton and twin pregnancies (p<0.001). Thymus size in monochorionic and dichorionic fetuses did not differ. If only comparing fetuses after 37+0 weeks of pregnancy, the difference in the TTR was no longer significant. In addition, an association between prematurity and larger thymus was established. However, the effect size of this association was small.

The results indicate that factors related to preterm birth, rather than the number of fetuses, are likely to influence the TTR. Utilizing fetal thymus size in ultrasound screening may hold potential for predicting adverse events like preterm birth in both singleton and twin pregnancies, but only in addition to other variables.