Thymic-thoracic ratio in fetuses with trisomy 21, 18 or 13

Area of Science:

• Fetal Medicine
• Immunology
• Genetics

Background:

• Thymic size is a crucial indicator of immune system development.
• Aneuploidies such as trisomy 21, 18, and 13 can impact fetal development.
• Understanding thymic development in trisomic fetuses is essential for predicting immune function.

Purpose of the Study:

• To evaluate thymic size, using the thymic-thoracic ratio (TT-ratio), in fetuses diagnosed with trisomy 21, 18, or 13.
• To compare thymic size in aneuploid fetuses with that of normal fetuses.
• To investigate the potential influence of intrauterine growth restriction (IUGR) on thymic size in these fetuses.

Main Methods:

• The TT-ratio was measured in 65 fetuses with trisomy 21 (n=30), 18 (n=19), or 13 (n=16) between 15 and 36 weeks' gestation.
• Fetuses were categorized into appropriate-for-gestational age (AGA) and IUGR groups.
• Measurements were compared against reference ranges established from 302 normal fetuses.

Main Results:

• A reduced TT-ratio was observed in 27.7% of aneuploid fetuses.
• Fetuses with trisomy 18 and 21 showed significantly smaller TT-ratios compared to normal fetuses (mean 0.38 and 0.40, respectively).
• Fetuses with trisomy 13 did not exhibit a significantly different TT-ratio; however, both AGA and IUGR fetuses showed reduced thymic size.

Conclusions:

• Trisomy 18 and 21 are associated with reduced fetal thymic size, suggesting in utero thymic involution.
• IUGR may contribute to smaller thymic size in trisomy 18 fetuses.
• The findings suggest a potential link between reduced thymic size in trisomy 21 and the observed immune dysfunction in Down syndrome.