What Can Be Seen on Prenatal Ultrasound?
Down syndrome, also known as Trisomy 21,
is one of the most common chromosomal conditions identified during pregnancy.
Many parents ask:
“Can ultrasound diagnose Down syndrome?”
The short answer is:
Ultrasound cannot diagnose Down syndrome.
It can only identify findings that may increase or decrease risk.
1️⃣ What Causes Down Syndrome?
Down syndrome occurs when a fetus has:
- An extra copy of chromosome 21
This leads to characteristic physical features
and may be associated with certain medical conditions.
It is a chromosomal condition,
not caused by something done during pregnancy.
2️⃣ Structural Findings on Ultrasound
Some fetuses with Down syndrome show structural differences.
The most important ones include:
🔹 Cardiac defects
- Atrioventricular septal defect (AVSD)
- Ventricular septal defect (VSD)
Heart abnormalities are among the most significant findings.
🔹 Gastrointestinal findings
- Duodenal atresia (“double bubble” sign)
🔹 Central nervous system findings
- Increased nuchal fold (NF)
- Mild ventriculomegaly
3️⃣ Soft Markers
Soft markers are subtle ultrasound findings
that may be more common in fetuses with Down syndrome.
Common soft markers include:
- Increased nuchal fold
- Hypoplastic nasal bone
- Short femur or humerus
- Echogenic bowel
- Mild pyelectasis
Important:
A soft marker alone does not mean the fetus has Down syndrome.
Risk depends on the overall clinical context.
4️⃣ When Does It Become Concerning?
The level of concern increases when:
- Multiple soft markers are present
- Structural abnormalities are detected
- Prior screening (NIPT or serum screening) shows high risk
If ultrasound findings are isolated and screening tests are low risk,
the overall probability may remain low.
5️⃣ Role of Screening vs Diagnosis
Ultrasound is a risk assessment tool, not a diagnostic test.
Definitive diagnosis requires:
- NIPT (non-invasive screening, high sensitivity)
- Amniocentesis or CVS (diagnostic testing)
6️⃣ Important Counseling Perspective
When discussing Down syndrome findings:
- Avoid definitive statements based on ultrasound alone
- Provide balanced risk interpretation
- Integrate prior screening results
Most importantly:
Ultrasound findings must be interpreted as part of a complete clinical picture.
Key Takeaways
- Down syndrome is caused by an extra chromosome 21
- Ultrasound cannot diagnose it, only suggest risk
- Structural abnormalities carry more weight than isolated soft markers
- Multiple findings increase risk more than a single marker
- Final diagnosis requires chromosomal testing
