💜 A personal note from Dr Joel: This article is written from the heart — because we lived it. When our daughter was 20 weeks in the womb, her anomaly scan picked up an ARSA. The words "soft marker for chromosomal abnormality" sent our world spinning. We went through the anxiety, the wait, the amniocentesis. She is now a healthy, thriving little girl. I'm writing this so that other parents who hear those same words have somewhere to turn for honest, clear information — without the panic spiral.
The right subclavian artery is one of the major blood vessels that branches off the aorta (the main artery from your heart) to supply blood to the right arm. In most people, it follows a predictable anatomical path. In some individuals — around 0.5–1.5% of the general population — this artery takes an unusual route, crossing behind the oesophagus (food pipe) rather than in front of it. This variant is called an Aberrant Right Subclavian Artery, or ARSA.
In plain English: it's a blood vessel that went a slightly different way. Most people who have it never know, because it causes no symptoms and requires no treatment.
ARSA is typically detected during the mid-trimester anomaly scan (18–22 weeks), when a sonographer examines the fetal heart and great vessels in detail. With modern high-resolution ultrasound, sonographers can now visualise the aortic arch and its branches clearly enough to detect this variant — something that wasn't routinely possible a decade ago.
The finding is documented, and parents are told that ARSA is a "soft marker" for chromosomal abnormality. Those three words — soft marker for chromosomal abnormality — are among the most anxiety-inducing phrases in obstetrics. Here's what they actually mean.
A soft marker is a finding on ultrasound that, by itself, is usually a normal variant — but is slightly more common in babies with certain chromosomal conditions (particularly Trisomy 21, or Down syndrome) than in the general population. Other common soft markers include a short femur, echogenic bowel, choroid plexus cysts, and a sandal gap toe.
Crucially: a soft marker is not a diagnosis. It is not even a strong predictor on its own. It is a finding that prompts a closer look.
Let's unpack that last number, because it's the most important one for most parents reading this.
Yes, ARSA is more common in Down syndrome babies — roughly 24% of babies with Trisomy 21 have ARSA. But that relationship works the other way around too: the vast majority of babies with ARSA do not have Trisomy 21. When ARSA is the only finding (no other soft markers, no structural abnormalities, normal first-trimester screening), the risk of chromosomal abnormality is extremely low.
A 2023 study of 121 cases of isolated ARSA found zero chromosomal abnormalities. A 2024 meta-analysis similarly found the likelihood ratio for Down syndrome in isolated ARSA was effectively zero.
✅ The bottom line: If your baby's ARSA is an isolated finding — no other markers, normal NIPT or combined screening — current evidence strongly suggests the risk of chromosomal abnormality is very low. Many centres no longer recommend invasive testing for isolated ARSA alone.
Amniocentesis is the gold-standard diagnostic test for chromosomal abnormalities. It involves taking a small sample of amniotic fluid (which contains fetal cells) and analysing the chromosomes directly. Unlike NIPT (a blood test), amniocentesis provides a definitive chromosomal result.
Whether amniocentesis is recommended for ARSA depends on:
The practice of recommending amniocentesis for isolated ARSA has evolved considerably. Many international centres — including the Fetal Medicine Foundation — have moved toward a more conservative approach, particularly when NIPT results are low-risk. Other centres still offer it, partly because the evidence base for truly isolated ARSA is still accumulating.
💡 On amniocentesis safety: Modern amniocentesis carries a procedure-related pregnancy loss risk of approximately 0.1–0.3% (1 in 300–1,000) in experienced hands. This risk-benefit calculation matters when the underlying risk being tested for is itself very low. Discuss this openly with your fetal medicine specialist.
In the vast majority of cases: nothing special. Most children with ARSA are entirely asymptomatic and require no treatment. A small minority may experience:
If symptomatic, ARSA can be surgically corrected, though intervention is rarely needed. Your paediatrician may request a fetal echocardiogram and will monitor feeding and growth in the newborn period.
I know exactly what that room feels like. The cold gel, the prolonged silence from the sonographer, the careful phrasing that tells you something has been found without quite telling you what it means. The drive home. The late-night searching.
Here is what I wish someone had told us clearly:
Our daughter is proof that this story has a happy ending. We hope yours does too.
References
Scala C et al. Isolated aberrant right subclavian artery: systematic review and meta-analysis. Ultrasound Obstet Gynecol. 2015;45(6):642–51
Papadopoulos G et al. Isolated ARSA and chromosomal abnormalities: cohort study 2023. PMC9975173
Fetal Medicine Foundation (fetalmedicine.org): Soft Markers Guidelines
Agathokleous M et al. Association of aberrant right subclavian artery with Down syndrome. PMC10411214
Frontiers in Pediatrics: ARSA Postnatal Outcomes (2022). doi:10.3389/fped.2022.895562
ACOG Practice Bulletin No. 226: Screening for Fetal Chromosomal Abnormalities (2020, reaffirmed 2023)