Normal Fetal Heart/Thorax Circumference Ratio Ultrasound
The heart circumference to thorax circumference (HC/TC) ratio is a sonographic measurement obtained during fetal cardiac evaluation to assess relative cardiac size within the chest. It provides a gestational-age-independent index, making it a practical screening tool across the second and third trimesters. Accurate assessment of this ratio is essential for identifying fetal cardiomegaly and guiding further workup.
Normal Reference Values
| Measurement |
|---|
| <0.5 |
Clinical Significance
A normal HC/TC ratio is less than 0.5, indicating that the fetal heart occupies less than half of the thoracic circumference at the level of the four-chamber view. When this ratio equals or exceeds 0.5, cardiomegaly should be suspected and further evaluation pursued. Because the ratio is relatively stable across gestational ages, a single threshold simplifies clinical decision-making.
Fetal cardiomegaly detected by an elevated HC/TC ratio carries important prognostic implications. It may reflect primary cardiac pathology, extrinsic compression, or a hyperdynamic circulatory state. Early identification allows timely referral for fetal echocardiography and multidisciplinary counseling.
- Ebstein anomaly — severe tricuspid regurgitation causing right heart dilatation
- Fetal hydrops — high-output cardiac failure with global cardiomegaly
- Severe fetal anemia (e.g., Rh isoimmunization, parvovirus B19)
- Arteriovenous malformation or sacrococcygeal teratoma — high-output state
- Cardiomyopathy — structural or metabolic causes (e.g., diabetic fetopathy)
Reference: Gandhi JG. Fetal Cardiology for Obstetricians. Jaypee Brothers Publishers. (2007).
Imaging Notes
The HC/TC ratio is measured on 2D ultrasound at the standard four-chamber view, obtained at the level of the fetal cardiac apex with the spine positioned posteriorly. Both circumferences are traced using the outer rib margin for the thorax and the outer cardiac border for the heart, or alternatively calculated from the respective diameters assuming a circular cross-section (circumference = π × diameter). Consistency in the measurement plane is critical — oblique sections artificially enlarge or reduce both structures and introduce error.
Optimal image quality requires a true axial plane with the heart filling approximately one-third of the thoracic area in a normal fetus. Color Doppler should not be active during circumference tracing, as it may expand the apparent cardiac borders. If cardiomegaly is suspected on screening, dedicated fetal echocardiography with functional assessment is recommended.