The development of the fetal thymus: An in utero sonographic evaluation

Objectives: To establish a nomogram for fetal thymus size during gestation. Methods The study is a prospective, cross-sectional evaluation of 403 male and female fetuses between 14 and 38 weeks of normal singleton pregnancies. Measurements of fetal thymus size were performed by high-resolution transvaginal ultrasonography between 14 and 17 weeks' gestation, and by transabdominal ultrasonography after 18 weeks' gestation. Results: Adequate thymus size measurements were obtained in all 403 fetuses. Thymus size as a function of gestational age was expressed by the regression equation: (square root) thymus size (mm)= -39.39 + 4.41 × gestational age (weeks). The correlation coefficient, r = 0.965, was found to be highly statistically significant (p <0.0001). The normal mean and the 90% prediction limits were defined. Conclusion The present data offer the normal range of fetal thymus size from early stages of gestation that may allow intrauterine assessment of its development. It may be helpful in the prenatal diagnosis of thymic pathologies.