We investigated the reliability of fetal thymus measurement during first-trimester screening, and associated fetal thymus size with crown-rump length, maternal diseases and fetal outcome.
In a retrospective cohort of 971 normal singleton first-trimester fetuses, we measured the anterior-posterior diameter of the thymus in a midsagittal plane in 767 fetuses. The intra-observer and inter-observer reliabilities were tested by intra-class correlation coefficient. We correlated thymus size with fetal crown-rump length, and investigated its association with maternal diseases (diabetes mellitus, rheumatic disorders, hypertension and coagulation disorders) and fetal outcome (small for gestational age, preterm birth and umbilical artery pH) using regression analyses.
The intra-observer and inter-observer reliabilities of fetal thymus measurement were excellent (intra-class correlation coefficient 0.926, 95% CI 0.745-0.981 and 0.945, 95% CI 0.886-0.993, respectively). A linear relationship was found between crown-rump length and thymus size (β = 0.023, p = 0.001). Pregnancies affected by maternal diabetes had a decreased fetal thymus size (β = -0.209, p = 0.001), whereas in pregnancies affected by maternal rheumatic disease the thymus size was increased (β = 0.285, p < 0.001). Fetal thymus size was not associated with maternal hypertension or maternal coagulation disorders. There was a positive association between preterm birth and fetal thymus size (p < 0.001).
Measurement of first-trimester thymus size is reliable. Fetal thymus size has a linear correlation with crown-rump length. Maternal diabetes, rheumatic disease and preterm birth appear to have an association with fetal thymus size.