Normal Fetal Thorax/Abdomen Circumference Ratio on Ultrasound
The thorax-to-abdomen circumference (TC/AC) ratio is a prenatal ultrasound measurement that compares the circumference of the fetal chest to that of the abdomen at standardized axial planes. It serves as an objective indicator of thoracic cage development relative to overall fetal size. Accurate assessment of this ratio is essential for identifying thoracic hypoplasia, which predicts pulmonary hypoplasia and perinatal respiratory compromise.
Normal Reference Values
| Measurement |
|---|
| >0.77 |
Clinical Significance
A TC/AC ratio of >0.77 is considered normal. Values falling below this threshold raise concern for a disproportionately small thorax, which may result in restricted lung growth and life-threatening pulmonary hypoplasia at birth. This ratio is particularly valuable when evaluating fetuses with suspected skeletal dysplasia, as thoracic size is a major determinant of lethality.
Short rib syndromes are among the most important conditions identified by a reduced TC/AC ratio. In Short Rib Polydactyly Syndrome Type 2 (Majewski syndrome), the severely shortened ribs produce a narrow, bell-shaped thorax with a markedly reduced TC/AC ratio, typically incompatible with postnatal life. Other conditions that may reduce this ratio include thanatophoric dysplasia, asphyxiating thoracic dystrophy (Jeune syndrome), and Ellis–van Creveld syndrome.
- Short rib polydactyly syndromes (Types 1–4)
- Thanatophoric dysplasia
- Asphyxiating thoracic dystrophy (Jeune syndrome)
- Ellis–van Creveld syndrome
- Campomelic dysplasia
Pitfall: Asymmetric fetal positioning or oblique imaging planes can artificially reduce measured thoracic circumference; always confirm measurements at the standard four-chamber cardiac plane and abdomen at the level of the stomach and umbilical vein confluence.
Reference: Jutur PS, Kumar CP, Goroshi S. Case report: Short rib polydactyly syndrome – type 2 (Majewski syndrome). Indian J Radiol Imaging. 2010;20 (2): 138-42.
Imaging Notes
On ultrasound, the thoracic circumference is measured at the level of the four-chamber cardiac view, tracing around the outer rib margins in an axial plane. The abdominal circumference is measured at the level where the umbilical vein meets the portal sinus, with the stomach visible. Both measurements should be obtained with the fetus in a neutral position to minimize measurement error. The ratio is then calculated by dividing TC by AC; a value above 0.77 indicates a normally proportioned thorax.
Three-dimensional ultrasound can supplement 2D assessment by providing lung volume estimates, but the TC/AC ratio remains a practical, widely available first-line metric in routine obstetric practice. Serial measurements may be helpful when thoracic narrowing is suspected but not definitively below threshold at a single examination.