The Nuchal Translucency (NT) Scan Explained
What the measurement is, what the numbers mean, and what a “higher chance” result actually does and does not tell you — written to help you understand your NHS screening.
The nuchal translucency scan measures a small pocket of fluid at the back of your baby’s neck, present in every pregnancy at this stage. Combined with two blood markers and your age, that measurement produces an estimate of the chance that your baby has Down’s syndrome, Edwards’ syndrome or Patau’s syndrome. In the UK it forms part of the combined screening test, offered free on the NHS between roughly 11 and 14 weeks.
Combined screening is an NHS programme, not simply a scan. It requires accredited nuchal measurement, blood samples processed by a linked laboratory, and validated risk-calculation software that combines all of it — then a care pathway for whatever the result shows. Your NHS 12-week appointment provides all of that, free. We explain it here so you can go into it informed; if you are looking to book an NT scan privately, the honest answer is that your NHS scan is the right place to have it.
What is measured, and when
Every baby has a fluid-filled space beneath the skin at the back of the neck in the first trimester. Measuring its depth is only meaningful within a narrow window: the scan is performed when the crown–rump length is between about 45mm and 84mm, corresponding to roughly 11 to 14 weeks. Before that the space is too small to measure reliably; after it, the fluid naturally reabsorbs and the measurement loses its meaning.
Precision matters enormously here. The baby has to be in a specific position, the image magnified to a standard, and the calipers placed exactly — which is why sonographers performing NT measurements work to a formal accreditation and are audited on their measurements. It is usually done abdominally, occasionally transvaginally if the view demands it, and takes around 20 to 30 minutes. Sometimes the baby will not cooperate and you are asked to walk around and come back.
Two blood markers are measured alongside it — PAPP-A and free beta-hCG — and these, your age, and the NT measurement are combined by software into a single result. The scan alone does not produce the answer.
What NT measurements mean
The measurement is reported in millimetres, and it is always interpreted against gestational age — the same number means different things at 11 weeks and at 14. As a general guide, measurements under about 3.5mm fall within the usual range, while a measurement above 3.5mm is associated with an increased chance of chromosomal conditions and of some heart and structural differences, and usually prompts additional assessment regardless of what the combined result says.
The crucial thing to hold onto: an increased measurement is not a diagnosis. Plenty of babies with a raised NT measurement are entirely healthy, and the measurement is one input among several. Equally, a normal measurement does not guarantee anything — screening estimates chance, it does not confirm or exclude.
Results are given as a chance, written as a ratio such as 1 in 500 or 1 in 100. The NHS describes a result as a higher chance when it is 1 in 150 or greater, and a lower chance below that. A “higher chance” result of 1 in 100 still means that in 100 pregnancies with the same result, around 99 babies would not have the condition — a framing that is worth remembering if you receive one.
Screening is not diagnosis
This single idea explains almost everything about how the pathway works. A screening test estimates how likely something is. A diagnostic test determines whether it is actually present. The combined test screens; it never diagnoses.
If you receive a higher-chance result, you will be offered a choice of next steps, and none of them is obligatory:
- NIPT — a maternal blood test analysing placental DNA. Far more accurate than combined screening, but still a screening test, not a diagnosis. Our guide to NIPT explains it.
- Chorionic villus sampling (CVS) — a diagnostic test sampling placental tissue, usually from 11 to 14 weeks.
- Amniocentesis — a diagnostic test sampling amniotic fluid, usually from 15 weeks.
- Declining further testing — an entirely legitimate choice, and one many people make.
CVS and amniocentesis give a definitive answer but carry a small risk of miscarriage, which is why NIPT is now commonly offered first. Whichever route you take, your NHS screening midwife is the person to talk it through with — that conversation is part of the programme.
Where a private scan fits in this window
We do not replicate NHS screening, but the 11 to 14 week window is one where people sometimes want an additional look — for dating certainty, for reassurance between appointments, or because an NHS appointment is further away than feels comfortable. A dating scan confirms viability, measures the baby and establishes dates accurately.
What we will not do is imply that such a scan tells you anything about chromosomal conditions. It does not, and any clinic suggesting otherwise is overselling. If screening is what you want, the NHS combined test is both better and free — and worth attending.
Your questions answered
What is a normal nuchal translucency measurement?
When is the NT scan done?
Does a high NT measurement mean my baby has Down’s syndrome?
What does a result of 1 in 150 mean?
Can I have an NT scan privately at IUS London?
Is the NT scan compulsory?
What is the difference between the NT scan and the anomaly scan?
Understanding your screening
How non-invasive prenatal testing works, and why it is still screening.
Read the guide →
18–21 WeeksThe Anomaly ScanWhat the mid-pregnancy scan checks, structure by structure.
Read the guide →
8–12 Weeks8 to 12 Weeks, Week by WeekWhat changes across the window the NT scan sits in.
Read the guide →
Dating and reassurance in the same window
We do not provide NHS screening — but if you want dating certainty or a look between NHS appointments, that we can do, with findings explained on screen and a written report usually within two hours.
Kensington, London W8 4ED
3 mins from Notting Hill Gate
Screening thresholds, timing and terminology follow NHS guidance on screening for Down’s, Edwards’ and Patau’s syndromes and NHS guidance on the 12-week scan. This article is general information, not individual medical advice — your screening results should be discussed with your NHS midwife or screening team, who provide counselling as part of the programme.
Written and clinically reviewed by the HCPC-registered sonographers who perform this scan at IUS London — a CQC-registered diagnostic ultrasound clinic (Provider ID 1-2775844974). Your own findings are explained to you at the scan and set out in your report.
Author: Yianni Kiromitis, Senior Sonographer, HCPC RA38415
Medically reviewed: 1 August 2026
