From the Research
Different sized atria on a prenatal ultrasound are most likely caused by congenital heart defects, and the primary recommendation is to proceed with further evaluation using a fetal echocardiogram and consultation with a pediatric cardiologist. The causes of this condition can be multifaceted, including structural heart abnormalities such as hypoplastic left heart syndrome, tricuspid or mitral valve abnormalities, or atrial septal defects 1. Other possibilities include arrhythmias that cause one atrium to dilate, cardiac tumors that restrict atrial space, or vascular anomalies affecting blood flow to or from the heart. Sometimes, the difference may be a normal variant, especially if mild, as the right atrium is naturally slightly larger than the left in fetal circulation.
When this finding is identified, it is crucial to assess the degree of size discrepancy, associated cardiac abnormalities, and gestational age to determine the significance and potential impact on the fetus. The diagnostic performance of fetal echocardiography is high, with sensitivity and specificity of 100% in detecting fetal cardiac abnormalities, as shown in a study comparing fetal echocardiography with fetal cardiovascular MRI using Doppler ultrasound gating 2.
The underlying mechanism typically involves abnormal blood flow patterns or pressure differences between the chambers, causing one atrium to enlarge or the other to remain underdeveloped. A study on the prenatal diagnosis of congenital heart defects using echocardiography highlights the importance of early detection and the role of fetal echocardiography in identifying and characterizing congenital heart diseases 3.
Given the potential implications for fetal and neonatal health, careful monitoring throughout pregnancy is warranted, and further diagnostic tests such as genetic testing may be recommended to identify any associated genetic abnormalities 1. The use of fetal MRI, as discussed in a study on the contribution of fetal magnetic resonance imaging in fetuses with congenital heart disease, can also provide additional information and alter prognosis, counseling, or management in approximately one-third of the fetuses 4.
In terms of diagnostic approaches, a comparative study on fetal cardiovascular assessment using Doppler ultrasound gated MRI and echocardiography found that both methods have high diagnostic performance, but fetal echocardiography remains the primary diagnostic tool for congenital heart defects 2. A meta-analysis on fetal echocardiography for congenital heart disease diagnosis also supports the use of fetal echocardiography as a powerful tool for the diagnosis of CHD, with a moderate sensitivity of 68.1% and a favorable specificity of 99.9% 5.
Overall, the identification of different sized atria on a prenatal ultrasound necessitates a comprehensive evaluation to determine the underlying cause and potential implications for fetal health, emphasizing the importance of fetal echocardiography and consultation with a pediatric cardiologist.