Assessing the risk of Down syndrome is one of the most important evaluations performed during the first trimester of pregnancy.
It is mainly carried out during the 11–12-week ultrasound scan, together with specific blood tests, and estimates the probability that the baby may have certain chromosomal abnormalities.
In this article, we explain what is assessed, how the results are interpreted and what the next steps may be depending on the outcome.
What is the Down syndrome risk assessment?
The Down syndrome risk assessment forms part of the combined first-trimester screening, a test that provides an estimate of risk rather than a definitive diagnosis.
This assessment combines:
- 11–12-week ultrasound scan.
- Maternal blood test.
- Maternal clinical information (age, weight and medical history).
The aim is to assess the risk of chromosomal abnormalities such as:
- Down syndrome (Trisomy 21).
- Edwards syndrome (Trisomy 18).
- Patau syndrome (Trisomy 13).
What is evaluated during the 12-week ultrasound?
The first-trimester ultrasound does much more than simply confirm the pregnancy.
During this scan, the following are assessed:
- Fetal viability (heartbeat).
- Gestational age.
- Basic fetal anatomy.
- Possible structural abnormalities.
Key ultrasound markers
Nuchal translucency
Nuchal translucency (NT) is one of the most important ultrasound markers.
It refers to a small collection of fluid at the back of the baby’s neck that, when increased, may be associated with:
- Chromosomal abnormalities.
- Congenital heart defects.
Important
An increased nuchal translucency does not confirm a diagnosis. It is simply an indicator of increased risk.
Other ultrasound markers evaluated
In addition to nuchal translucency, the following are assessed:
- Nasal bone.
- Ductus venosus.
- Tricuspid valve.
The scan also evaluates fetal organs, including:
- Skull.
- Limbs.
- Stomach.
- Bladder.
- Abdomen.
What is combined first-trimester screening?
To improve accuracy, the ultrasound findings are combined with a maternal blood test measuring:
- PAPP-A.
- Free beta-hCG.
These results, together with the ultrasound measurements and maternal characteristics, are used to calculate the risk.
This screening can detect up to 85% of chromosomal abnormalities before the 13th week of pregnancy.
How are the results interpreted?
The result is expressed as a probability.
For example:
- Low risk: values higher than 1 in 270.
- High risk: values lower than 1 in 270.
It is important to understand that:
- It is not a diagnosis.
- It cannot confirm or completely rule out a chromosomal condition.
- It simply indicates whether further testing is recommended.
What happens if the risk is high?
If the screening indicates a high risk, your specialist may recommend:
Non-invasive prenatal testing (NIPT)
- Maternal blood test.
- High accuracy.
- No risk to the baby.
Invasive diagnostic tests (when indicated)
- Amniocentesis.
- Chorionic villus sampling (CVS).
These tests can confirm the diagnosis.
How reliable is the Down syndrome risk assessment?
Combined first-trimester screening is a valuable and widely used tool, but it is important to remember that:
- It is a screening test, not a diagnostic test.
- False-positive and false-negative results are possible.
- Results should always be interpreted by an experienced specialist.
Conclusion
Down syndrome risk assessment is a key first-trimester screening test that helps identify pregnancies at increased risk of chromosomal abnormalities at an early stage.
A properly performed 12-week ultrasound, combined with the appropriate blood tests, provides valuable information that helps parents and healthcare professionals make informed decisions and, if necessary, proceed with more accurate diagnostic tests.
If you are pregnant or would like comprehensive prenatal care from the very beginning, Clínica Fertia offers specialist prenatal diagnosis services in Málaga.
Book your appointment and let our team support you throughout every stage of your pregnancy.
Frequently Asked Questions about the 12-Week Ultrasound
Can the 12-week ultrasound confirm whether my baby has Down syndrome?
No. This examination estimates the level of risk but cannot definitively confirm or rule out Down syndrome. If the result indicates a high risk, additional tests are usually recommended to confirm the diagnosis.
What is the difference between combined screening and the non-invasive prenatal test (NIPT)?
Combined screening estimates the risk using the ultrasound findings, maternal blood test and maternal characteristics. The non-invasive prenatal test (NIPT) analyses fetal DNA circulating in the mother’s blood and offers greater accuracy, although it is still considered a screening test.
Do I need to fast before the first-trimester blood test?
It depends on your clinic’s protocol. You should always follow the instructions provided by your healthcare team before the test.
What happens if one of the ultrasound markers cannot be measured accurately?
In some cases, the ultrasound may need to be repeated or complemented with additional tests. Image quality, fetal position and the exact stage of pregnancy can all influence the examination.
Does the mother’s age affect the Down syndrome risk calculation?
Yes. Maternal age is one of the factors included in the combined first-trimester screening, together with the blood test results and ultrasound findings.
Can this assessment be performed in any pregnancy, even without previous risk factors?
Yes. First-trimester combined screening is recommended for all pregnancies because it helps assess the risk of chromosomal abnormalities even when there are no previous risk factors or symptoms.
