NT scan (nuchal translucency)
Measures fluid at the back of the baby's neck to estimate the chance of Down's syndrome and two other conditions. A risk, not a diagnosis.
When: 11 weeks to 13 weeks and 6 days — and not outside it
Screening test — gives a chance, not an answer
Also called: Nuchal scan · 11-13 week scan · First-trimester screening
What it is
Every baby has a small pocket of fluid at the back of the neck. The NT scan measures how thick it is. More fluid than usual is associated with a higher chance of Down's syndrome and of two other chromosomal conditions, Edwards' and Patau's syndromes.
This is a screening test, not a diagnosis. That distinction is the most important thing on this page. It gives a number — a chance, like 1 in 800 — and it cannot tell you whether your baby actually has a condition. Only a diagnostic test can do that, and one is offered if the screening result warrants it.1
The measurement is usually combined with a blood test taken around the same time, which is where the term *combined screening* comes from. Together they give a more useful number than either alone.
Why the timing is strict
The fluid can only be measured between 11 weeks and 13 weeks and 6 days. Before 11 weeks the baby is too small to measure reliably; after 13 weeks and 6 days the fluid is normally reabsorbed and there is nothing left to measure. Miss the window and this test is simply not available to you — other screening options exist, and the doctor will talk you through them.2
What a 'high risk' result means
It means the chance is higher than a set threshold — not that anything is wrong. Most women who receive a higher-risk result go on to have a baby with no chromosomal condition at all. What it does is open the door to a diagnostic test that can give a definite answer, and that decision is entirely yours.1
What it cannot tell you
It does not look for physical abnormalities in the way the anomaly scan does, and it does not screen for every chromosomal condition. A normal result lowers the chance of the specific conditions screened for; it does not rule out everything.
No scan or screening test can rule out every condition. What each one looks at, what it found, and what it means for you is a conversation with the doctor who performed it — this page explains the tests, it does not interpret your results.
References
- American College of Obstetricians and Gynecologists Committee on Practice Bulletins—Obstetrics, Committee on Genetics, Society for Maternal-Fetal Medicine. Screening for Fetal Chromosomal Abnormalities: ACOG Practice Bulletin, Number 226. Obstetrics and Gynecology. 2020;136(4):e48-e69. PMID 32804883. doi:10.1097/AOG.0000000000004084The ACOG guideline on chromosomal screening — that every screening test gives a risk rather than a diagnosis, and that a diagnostic test is needed to confirm a positive result.
- Salomon LJ, Alfirevic Z, Bilardo CM, Chalouhi GE, et al.. ISUOG practice guidelines: performance of first-trimester fetal ultrasound scan. Ultrasound in Obstetrics & Gynecology. 2013;41(1):102-13. PMID 23280739. doi:10.1002/uog.12342The international guideline on the first-trimester scan, including the 11 to 13 weeks 6 days window in which nuchal translucency can be measured and dating is most accurate.
Sources are listed for the specific statements they support. They are published research and guidance, not a substitute for a consultation — what applies to you is decided by a doctor who has assessed you.