04/12/2025
The primary importance of the right subclavian artery (RSA) during the second trimester ultrasound screen lies in detecting the presence of an Aberrant Right Subclavian Artery (ARSA).
ARSA is an anatomical variant where the RSA arises as the last branch of the aorta and usually runs behind the trachea and esophagus to reach the right arm. While this is an asymptomatic finding in the majority of cases and is present in about 1-2% of the general population, its detection during a prenatal ultrasound is significant for the following reasons:
Marker for Chromosomal Abnormalities: ARSA is considered a "soft marker" for aneuploidies, particularly Down syndrome (Trisomy 21).
The incidence of ARSA is significantly higher in fetuses with Down syndrome (reported as 19%–36%) compared to chromosomally normal fetuses (0.4%–1.5%).
Association with Other Anomalies: ARSA is also associated with an increased risk of other chromosomal defects and congenital cardiac defects.
Clinical Management: The detection of ARSA, especially when accompanied by other ultrasound findings or a high-risk first-trimester screening, prompts further investigation, such as genetic counseling and consideration of non-invasive prenatal testing (NIPT) or invasive testing (amniocentesis) to assess for chromosomal anomalies.
In summary, identifying an ARSA is a key component of the detailed fetal anatomy scan in the second trimester because it serves as a signpost that increases the suspicion for certain fetal abnormalities, most notably Down syndrome.
Would you like more detail on what an Aberrant Right Subclavian Artery (ARSA) is or its relationship to Down syndrome?
