Down syndrome is a common genetic disorder caused by an extra copy of chromosome 21. Early detection during pregnancy helps parents prepare and make informed decisions. This article explains which scans can detect Down syndrome and how they work.
Understanding Down Syndrome Screening
Down syndrome cannot be diagnosed with a standard ultrasound scan alone. Instead, specific screening tests and diagnostic procedures are used during pregnancy.
Key Scans and Tests for Down Syndrome Detection
- Nuchal Translucency (NT) Scan (11-14 weeks):
This ultrasound measures the fluid-filled space at the back of the fetus’s neck. A thicker NT measurement may indicate a higher risk for Down syndrome and other chromosomal abnormalities. It is part of the first-trimester combined screening test. - First-Trimester Combined Screening:
Combines NT measurement with maternal blood tests measuring pregnancy-associated plasma protein A (PAPP-A) and free beta-hCG. Together, they provide a risk estimate for Down syndrome. - Second-Trimester Quadruple (Quad) Test (15-20 weeks):
A blood test measuring four substances in the mother’s blood: alpha-fetoprotein (AFP), hCG, estriol, and inhibin-A. It helps assess the risk of Down syndrome. - Non-Invasive Prenatal Testing (NIPT):
A blood test analysing fetal DNA in the mother’s blood to detect chromosomal abnormalities, including Down syndrome, with high accuracy. - Diagnostic Tests:
If screening tests show high risk, diagnostic tests like Chorionic Villus Sampling (CVS) or Amniocentesis provide a definitive diagnosis by analysing fetal chromosomes.
Why Regular Ultrasound Alone Is Not Enough
Routine ultrasounds can show physical markers that raise suspicion but cannot confirm Down syndrome. Specialized scans combined with blood tests and NIPT offer reliable screening.
When Should These Scans Be Done?
Test/Scan |
Timing |
Purpose |
NT Scan + First Trimester Blood Test |
11 to 14 weeks |
Initial risk assessment |
Quad Test |
15 to 20 weeks |
Second trimester risk screening |
NIPT |
As early as 9 weeks |
High accuracy non-invasive screening |
CVS or Amniocentesis |
10-13 weeks (CVS) / 15-20 weeks (Amnio) |
Diagnostic confirmation |
The NT scan, combined with blood tests and NIPT, is the cornerstone of Down syndrome screening during pregnancy. For a definitive diagnosis, invasive diagnostic tests are necessary. Early screening and counselling empower parents with knowledge and choices.