title: Thymic-thoracic ratio in fetuses with trisomy 21, 18 or 13.
abstract: OBJECTIVE
  To assess thymic size expressed as the thymic-thoracic ratio (TT-ratio) in fetuses with trisomy 21, 18 or 13.
  
  
  METHODS
  The TT-ratio, the quotient of the anteroposterior thymic and the intrathoracic mediastinal diameter, was measured in 65 trisomic fetuses between 15 and 36 weeks' gestation, including 30 cases with trisomy 21, 19 with trisomy 18 and 16 with trisomy 13. In addition these 65 fetuses were divided into two groups, according to whether they showed growth that was appropriate-for-gestational age (AGA) (n = 39) or intrauterine growth restriction (IUGR) (n = 26). Measurements were compared with reference ranges from 302 normal fetuses.
  
  
  RESULTS
  The TT-ratio was low in 27.7% (n = 18) of the 65 fetuses with aneuploidy. In comparison to normal fetuses (mean TT-ratio, 0.44), those with trisomy 18 or 21 had a significantly smaller TT-ratio (mean, 0.38 (P < 0.001) and 0.40 (P < 0.05), respectively), while those with trisomy 13 did not (mean, 0.43). These values were not as low as those observed previously in fetuses with del.22q11, suggesting a mechanism involving accelerated thymic involution rather than primary thymic hypoplasia. Furthermore, the TT-ratio was significantly lower than normal in both AGA (P < 0.05) and IUGR (P < 0.001) fetuses.
  
  
  CONCLUSIONS
  Fetuses with trisomy 18 or 21, but not trisomy 13, have a small thymus, suggesting accelerated thymic involution in utero. IUGR may contribute to the reduced thymic size in trisomy 18 fetuses. Trisomy 21 fetuses seem to have additional factors leading to a small thymus which could be a possible confirmation of the reduced immune response observed in fetuses and neonates with Down syndrome.
authors.affiliation: Department of Obstetrics and Gynecology, Ludwig-Maximilians-University, Munich, Germany.
authors.forename: K
authors.initials: K
authors.lastname: Karl
authors.forename: K-S
authors.initials: KS
authors.lastname: Heling
authors.forename: A
authors.initials: A
authors.lastname: Sarut Lopez
authors.forename: G
authors.initials: G
authors.lastname: Thiel
authors.forename: R
authors.initials: R
authors.lastname: Chaoui
mesh_terms: D000328:Adult; D025063:Chromosome Disorders / Q000000981:diagnostic imaging* / Q000196:embryology / Q000473:pathology; D002882:Chromosomes, Human, Pair 13 / Q000000981:diagnostic imaging; D002887:Chromosomes, Human, Pair 18 / Q000000981:diagnostic imaging; D004314:Down Syndrome / Q000000981:diagnostic imaging* / Q000196:embryology / Q000473:pathology; D005260:Female; D005317:Fetal Growth Retardation / Q000000981:diagnostic imaging*; D005865:Gestational Age; D006801:Humans; D008297:Male; D009929:Organ Size; D011237:Predictive Value of Tests; D011247:Pregnancy; D011256:Pregnancy Outcome; D013909:Thorax / Q000000981:diagnostic imaging* / Q000196:embryology / Q000473:pathology; D013950:Thymus Gland / Q000000981:diagnostic imaging* / Q000196:embryology / Q000473:pathology; D014314:Trisomy / Q000473:pathology; D000073839:Trisomy 13 Syndrome; D016216:Ultrasonography, Prenatal