Introduction: The purpose was to compare thymus size measured during second trimester screening of fetuses who were subsequently small for gestational age at birth (weight below 10th percentile, SGA group) with fetuses with normal birth weight (control group). We hypothesized that measuring the fetal thymic-thoracic ratio (TT-ratio) might help predict low birth weight. Methods: Using three-vessel view echocardiograms from our archives, we measured the anteroposterior thymus size and the intrathoracic mediastinal diameter to derive TT-ratios in the SGA (n = 105) and control groups (n = 533) between 19+0 and 21+6 weeks of gestation. We analyzed the association between TT-ratio and SGA adjusted to the week of gestation using logistic regression. Finally, we determined the possible TT-ratio cut-off point for discrimination between SGA and control groups by means of receiver operating characteristics (ROC) curve analysis. Results: The TT-ratio was significantly higher in the SGA group than in the control group (p < 0.001). An increase of the TT-ratio by 0.1 was associated with a 3.1-fold increase in the odds of diagnosing SGA. We determined that a possible discrimination cut-off point between SGA and healthy controls was achieved using a TT-ratio of 0.390 (area under the ROC curve 0.695). Conclusion: An increased TT-ratio may represent an additional prenatal screening parameter that improves the prediction of birth weight below the 10th percentile. Prospective studies are now needed to evaluate the use of fetal thymus size as predictive parameter for adverse fetal outcome.

Unterscheider J, Daly S, Geary MP, Kennelly MM, McAuliffe FM, O’Donoghue K, et al. Optimizing the definition of intrauterine growth restriction: the multicenter prospective PORTO Study. Am J Obstet Gynecol. 2013 Apr;208(4):290.e1–6.
Unterscheider J, Daly S, Geary MP, Kennelly MM, McAuliffe FM, O’Donoghue K, et al. Definition and management of fetal growth restriction: a survey of contemporary attitudes. Eur J Obstet Gynecol Reprod Biol. 2014 Mar;174:41–5.
Gordijn SJ, Beune IM, Ganzevoort W. Building consensus and standards in fetal growth restriction studies. Best Pract Res Clin Obstet Gynaecol. 2018 May;49:117–26.
Gardosi J, Madurasinghe V, Williams M, Malik A, Francis A. Maternal and fetal risk factors for stillbirth: population based study. BMJ. 2013 Jan 24;346:f108.
Barker DJ. Adult consequences of fetal growth restriction. Clin Obstet Gynecol. 2006 Jun;49(2):270–83.
Lees C, Marlow N, Arabin B, Bilardo CM, Brezinka C, Derks JB, et al. Perinatal morbidity and mortality in early-onset fetal growth restriction: cohort outcomes of the trial of randomized umbilical and fetal flow in Europe (TRUFFLE). Ultrasound Obstet Gynecol. 2013 Oct;42(4):400–8.
Felker RE, Cartier MS, Emerson DS, Brown DL. Ultrasound of the fetal thymus. J Ultrasound Med. 1989 Dec;8(12):669–73.
Zalel Y, Gamzu R, Mashiach S, Achiron R. The development of the fetal thymus: an in utero sonographic evaluation. Prenat Diagn. 2002 Feb;22(2):114–7.
Cho JY, Min JY, Lee YH, McCrindle B, Hornberger LK, Yoo SJ. Diameter of the normal fetal thymus on ultrasound. Ultrasound Obstet Gynecol. 2007 Jun;29(6):634–8.
Chaoui R, Heling KS, Lopez AS, Thiel G, Karl K. The thymic-thoracic ratio in fetal heart defects: a simple way to identify fetuses at high risk for microdeletion 22q11. Ultrasound Obstet Gynecol. 2011 Apr;37(4):397–403.
Di Naro E, Cromi A, Ghezzi F, Raio L, Uccella S, D’Addario V, et al. Fetal thymic involution: a sonographic marker of the fetal inflammatory response syndrome. Am J Obstet Gynecol. 2006 Jan;194(1):153–9.
El-Haieg DO, Zidan AA, El-Nemr MM. The relationship between sonographic fetal thymus size and the components of the systemic fetal inflammatory response syndrome in women with preterm prelabour rupture of membranes. BJOG. 2008 Jun;115(7):836–41.
Yinon Y, Zalel Y, Weisz B, Mazaki-Tovi S, Sivan E, Schiff E, et al. Fetal thymus size as a predictor of chorioamnionitis in women with preterm premature rupture of membranes. Ultrasound Obstet Gynecol. 2007 Jun;29(6):639–43.
Nau TG, de Murcia KO, Möllers M, Braun J, Abhari RE, Steinhard J, et al. Foetal thymus size in pregnancies after assisted reproductive technologies. Arch Gynecol Obstet. 2018 Aug;298(2):329–36.
Dörnemann R, Koch R, Möllmann U, Falkenberg MK, Möllers M, Klockenbusch W, et al. Fetal thymus size in pregnant women with diabetic diseases. J Perinat Med. 2017 Jul 26;45(5):595–601.
Gasthaus CL, Schmitz R, Hammer K, Oelmeier de Murcia K, Falkenberg MK, Braun J, et al. Influence of maternal HIV infection on fetal thymus size. J Perinat Med. 2019 Dec 18;48(1):67–73.
Cromi A, Ghezzi F, Raffaelli R, Bergamini V, Siesto G, Bolis P. Ultrasonographic measurement of thymus size in IUGR fetuses: a marker of the fetal immunoendocrine response to malnutrition. Ultrasound Obstet Gynecol. 2009 Apr;33(4):421–6.
Ekin A, Gezer C, Taner CE, Solmaz U, Gezer NS, Ozeren M. Prognostic value of fetal thymus size in intrauterine growth restriction. J Ultrasound Med. 2016 Mar;35(3):511–7.
Brandt JS, Bastek JA, Wang E, Purisch S, Schwartz N. Second-trimester sonographic thymus measurements are not associated with preterm birth and other adverse obstetric outcomes. J Ultrasound Med. 2016 May;35(5):989–97.
Causevic M, Mohaupt M. 11beta-Hydroxysteroid Dehydrogenase Type 2 in pregnancy and preeclampsia. Mol Aspects Med. 2007 Apr;28(2):220–6.
Olearo E, Oberto M, Oggè G, Botta G, Pace C, Gaglioti P, et al. Thymic volume in healthy, small for gestational age and growth restricted fetuses. Prenat Diagn. 2012 Jul;32(7):662–7.
Nardozza LM, Caetano AC, Zamarian AC, Mazzola JB, Silva CP, Marçal VM, et al. Fetal growth restriction: current knowledge. Arch Gynecol Obstet. 2017 May;295(5):1061–77.
Pearse G. Histopathology of the thymus. Toxicol Patho. 2006;34(5):515–47.
Nasseri F, Eftekhari F. Clinical and radiologic review of the normal and abnormal thymus: pearls and pitfalls. Radiographics. 2010 Mar;30(2):413–28.
Choyke PL, Zeman RK, Gootenberg JE, Greenberg JN, Hoffer F, Frank JA. Thymic atrophy and regrowth in response to chemotherapy: CT evaluation. AJR Am J Roentgenol. 1987 Aug;149(2):269–72.
Gruver AL, Ventevogel MS, Sempowski GD. Leptin receptor is expressed in thymus medulla and leptin protects against thymic remodeling during endotoxemia-induced thymus involution. J Endocrinol. 2009 Oct;203(1):75–85.
Tagoma A, Haller-Kikkatalo K, Roos K, Oras A, Kirss A, Ilonen J, et al. Interleukin-7, T helper 1, and regulatory T-cell activity-related cytokines are increased during the second trimester of healthy pregnancy compared to non-pregnant women. Am J Reprod Immunol. 2019 Dec;82(6):e13188.
Majumdar S, Nandi D. Thymic atrophy: experimental studies and therapeutic interventions. Scand J Immunol. 2018 Jan;87(1):4–14.
Warby A, Amler S, Jacobi A, Hammer K, Möllmann U, Falkenberg M, et al. Imaging of fetal thymus in pregnant women with rheumatic diseases. J Perinat Med. 2014 Sep;42(5):635–9.
Figueras F, Eixarch E, Gratacos E, Gardosi J. Predictiveness of antenatal umbilical artery Doppler for adverse pregnancy outcome in small-for-gestational-age babies according to customised birthweight centiles: population-based study. BJOG. 2008 Apr;115(5):590–4.
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