An NT scan (nuchal translucency scan) is a first-trimester ultrasound that measures a small fluid-filled space at the back of a developing baby’s neck. This measurement, typically taken between 11 and 14 weeks of pregnancy, helps estimate the chance that a baby has certain chromosomal conditions like Down syndrome. It’s a screening test, not a diagnosis, meaning it identifies higher-risk pregnancies that may benefit from further testing.
What the Scan Measures
Every baby has a thin layer of fluid beneath the skin at the back of the neck during the first trimester. In babies with certain chromosomal differences, this fluid collection tends to be thicker than usual. The NT scan measures this space in millimeters.
To get an accurate reading, the sonographer needs to position the image very precisely. The baby must be seen in profile (a mid-sagittal view) with the head in a neutral position, in line with the spine. The image is then zoomed in so only the head and upper chest are visible. The measurement is taken between the skin and the soft tissue overlying the cervical spine, and the sonographer must distinguish between the baby’s skin and the surrounding amniotic membrane, which can look similar on ultrasound. If the baby is curled up or turned the wrong way, the technician may ask you to walk around or wait until the baby shifts position.
When It’s Done
The scan is performed when the baby measures between 45 and 84 mm from head to rump (crown-rump length), which corresponds roughly to 11 to 14 weeks of pregnancy. Before this window, the baby is too small for an accurate measurement. After it, the fluid naturally begins to be reabsorbed, making results unreliable.
What It Screens For
The primary conditions the NT scan helps screen for are three chromosomal abnormalities: Down syndrome (trisomy 21), trisomy 18, and trisomy 13. Down syndrome involves an extra copy of chromosome 21 and can cause intellectual disability along with heart and digestive complications. Trisomy 18 and trisomy 13 are more severe conditions that affect multiple organ systems and carry a much poorer prognosis.
A thicker-than-expected nuchal fold is also associated with congenital heart defects and skeletal problems, even when chromosomes are normal. So an increased measurement can prompt closer monitoring of the baby’s heart development later in pregnancy.
How Results Are Interpreted
There isn’t one universal cutoff that separates “normal” from “abnormal.” Instead, the measurement is compared against expected values for the baby’s size. Generally, a nuchal translucency under about 2.5 mm falls within the normal range for most gestational ages. Measurements between 2.5 and 3.5 mm are considered increased, and those above 3.5 mm (above the 99th percentile) carry the strongest association with adverse outcomes.
That said, a single millimeter reading alone doesn’t determine your risk. The NT measurement is plugged into a software algorithm along with several other factors: your age, the baby’s crown-rump length, whether the nasal bone is visible on ultrasound, and blood flow patterns in a specific fetal vein called the ductus venosus. This produces a combined risk estimate expressed as a ratio, like 1 in 10,000 (very low risk) or 1 in 150 (higher risk).
The Blood Test That Goes With It
The NT scan is most effective when paired with a blood draw, forming what’s called first-trimester combined screening. The blood test measures two markers. The first is PAPP-A, a protein produced by the placenta that tends to be lower than expected in pregnancies affected by Down syndrome. The second is hCG (human chorionic gonadotropin), a hormone that tends to be higher in affected pregnancies.
On its own, the NT scan detects roughly 70 to 77% of Down syndrome cases. When the ultrasound measurement and blood markers are combined with maternal age in the risk algorithm, the screening becomes more accurate. The combined approach carries a false-positive rate of about 5%, meaning that out of every 100 women flagged as higher risk, about 95 will ultimately have a baby without the screened conditions.
What a High-Risk Result Means
A screening result that comes back as higher risk does not mean your baby has a chromosomal condition. It means the probability is high enough to warrant further investigation. Most people who receive a high-risk NT result go on to have healthy pregnancies.
The next step is typically a diagnostic test. The two main options are chorionic villus sampling (CVS), which can be done as early as 10 to 13 weeks, and amniocentesis, usually performed after 15 weeks. Both involve collecting a small sample of placental or amniotic fluid cells to examine the baby’s actual chromosomes, giving a definitive answer rather than a probability. Some providers also offer non-invasive prenatal testing (NIPT), a blood test that analyzes fragments of fetal DNA circulating in the mother’s bloodstream. NIPT is highly accurate for the major trisomies but is still considered a screening test, not a diagnostic one.
Factors That Affect Accuracy
Several things can make the measurement less reliable. Fetal position is the biggest variable. If the baby’s neck is extended (head tilted back) or flexed (chin tucked down), the reading can be artificially increased or decreased. This is why strict imaging criteria exist and why the scan sometimes takes longer than you might expect.
The sonographer’s training also matters. NT measurement is operator-dependent, and many professional organizations require specific certification for practitioners who perform these scans. If you’re offered an NT scan, it’s reasonable to confirm that the provider holds this credential.
Pregnancies conceived through assisted reproductive techniques like IVF may also show slightly different NT patterns, which is something your provider factors into the risk calculation. And because the normal range of NT thickness changes as the baby grows, the measurement must always be interpreted relative to the baby’s crown-rump length on that specific day, not as a standalone number.