Normal Thorax/Abdomen Circumference Ratio on Ultrasound
The thorax circumference to abdomen circumference (TC/AC) ratio is a fetal biometric parameter measured on prenatal ultrasound to assess thoracic cage size relative to overall abdominal girth. This ratio is clinically important because an abnormally small thorax may indicate pulmonary hypoplasia and lethal skeletal dysplasia, which profoundly affect neonatal survival and perinatal counseling.
Normal Reference Values
| Measurement |
|---|
| >0.77 |
Clinical Significance
A TC/AC ratio of >0.77 is considered within the normal range. When the ratio falls below this threshold, it suggests a disproportionately small thoracic cage, raising concern for conditions in which pulmonary development is compromised. Pulmonary hypoplasia secondary to thoracic restriction carries a high risk of neonatal respiratory failure and death.
The TC/AC ratio is particularly valuable in the evaluation of skeletal dysplasias, where narrow chest deformity is a hallmark feature. A low ratio should prompt a thorough assessment for additional skeletal, renal, and soft-tissue anomalies. Clinicians should be aware that fetal position, oligohydramnios, and suboptimal imaging planes can affect measurement accuracy, potentially leading to over- or under-estimation of true thoracic size.
- Short rib polydactyly syndromes (e.g., Majewski syndrome, Saldino-Noonan)
- Thanatophoric dysplasia
- Asphyxiating thoracic dysplasia (Jeune syndrome)
- Campomelic dysplasia
- Ellis-van Creveld syndrome
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 obstetric ultrasound, the thorax circumference is measured at the level of the four-chamber cardiac view using the outer chest wall contour, while the abdomen circumference is obtained at the standard biometric plane through the fetal liver and stomach. Both measurements should be made in true axial cross-sections to ensure comparability. The ratio is then calculated by dividing TC by AC.
Careful attention to fetal lie and avoidance of oblique imaging planes are essential for reproducibility. Measurements are applicable across the second and third trimesters; serial assessment may be warranted when initial findings are borderline or when a skeletal dysplasia is suspected based on other biometric parameters.