Pregnancy Scans
NT Scan (Nuchal Translucency)
Screens for chromosomal conditions including Down syndrome
What is this scan?
The NT scan, or Nuchal Translucency scan, is a specialised first trimester ultrasound that measures the fluid-filled space at the back of your baby's neck (the nuchal translucency). It is performed in a precise window: between 11 weeks 2 days and 13 weeks 6 days of pregnancy, when the baby's CRL is between 45mm and 84mm. The measurement is combined with a blood test (the Double Marker — PAPP-A and free beta-hCG) and your age to calculate a personalised risk score for Down syndrome (Trisomy 21), Trisomy 18 (Edwards syndrome), and Trisomy 13 (Patau syndrome). It is a screening test, not a diagnostic test — it tells you the probability, not a definitive answer.
Why is it done?
The NT scan is done because chromosomal conditions like Down syndrome can be screened non-invasively in the first trimester, allowing parents to make informed decisions early in the pregnancy. The combined first trimester screen (NT + blood test + maternal age) detects approximately 90% of Down syndrome cases with a 5% false-positive rate. The scan also checks the baby's nasal bone (absent in some chromosomal conditions), early heart structure, and other markers. Missing the 11–14 week window means this specific test cannot be performed.
What to expect
The scan must be done within the specific window — you cannot delay it. The baby needs to be in the right position for the sonographer to get a precise measurement of the neck fluid; this sometimes requires some waiting, repositioning, or gentle movement. The scan takes 30–45 minutes. If the baby's position is not ideal through the abdominal probe, a transvaginal component may be added for clearer images. Blood is drawn the same day (or close to it) for the Double Marker test. Results of the combined risk calculation are usually available within a few days.
Normal findings
- ✓ NT measurement < 3.0mm is considered normal
- ✓ Combined first trimester risk < 1:300 is classified as low risk
- ✓ Nasal bone visible (absent nasal bone is a soft marker for Down syndrome)
- ✓ Normal early cardiac appearance
- ✓ Ductus venosus flow pattern normal
When to be concerned
- NT ≥ 3.5mm — significantly increased risk of chromosomal abnormality or structural heart defect
- NT 3.0–3.4mm — borderline; combined result and detailed counselling needed
- Combined risk result > 1:150 — classified as high risk; invasive testing (CVS or amniocentesis) may be offered
- Absent nasal bone — associated with Down syndrome
- Abnormal ductus venosus waveform — associated with cardiac and chromosomal anomalies
Frequently asked questions
What happens if the NT measurement is increased?
An increased NT does not mean your baby definitely has a chromosomal condition — it raises the probability. Your doctor will discuss the combined risk result with you and offer further testing options: NIPT (a highly accurate blood test from 10 weeks) or invasive testing (CVS or amniocentesis) which give definitive chromosomal answers. Increased NT can also indicate heart defects, even with normal chromosomes, so a fetal echocardiogram may be recommended.
Can the NT scan be repeated if the window is missed?
No. Once the baby is beyond 13 weeks 6 days (CRL > 84mm), the NT scan cannot be performed — the nuchal fluid is absorbed by the baby's lymphatic system. If you miss the window, second trimester screening (Triple or Quad Marker blood test, or NIPT) is the next option.
What is the difference between the NT scan and the TIFFA scan?
The NT scan is a first trimester screen (11–14 weeks) focused on chromosomal risk assessment using the nuchal translucency measurement. The TIFFA (anomaly) scan is a detailed second trimester scan (18–22 weeks) that checks all the baby's organs and structures for physical abnormalities. Both are important, but they serve different purposes.
Is the Double Marker blood test included with the NT scan?
The Double Marker (PAPP-A + free beta-hCG) is a separate blood test drawn on the same day or within a few days of the NT scan. Both results are entered into a risk calculation software along with your age, weight, and scan measurements to generate your combined risk score. Ask your doctor or the scanning centre whether it is included in the package.
Does a 'low risk' result mean my baby is 100% healthy?
A 'low risk' result means the statistical probability of a chromosomal condition is very low, but it is not a 100% guarantee. No screening test can rule out all conditions, but the NT scan combined with blood work is the most reliable first-trimester screening tool available.