An NT scan, short for nuchal translucency scan, is a prenatal ultrasound that measures a small pocket of fluid at the back of your baby’s neck during the first trimester. This measurement helps estimate the risk of Down syndrome and other genetic or structural conditions. It’s typically performed between 11 and 14 weeks of pregnancy and is one of the earliest screening tests available.
What the Scan Measures
Every developing baby has a thin layer of fluid beneath the skin at the back of the neck. The NT scan uses ultrasound to measure the thickness of this fluid collection in millimeters. A thicker-than-expected measurement suggests a higher chance that the baby has a chromosomal condition or structural difference, particularly involving the heart.
During the scan, the sonographer also checks for other early physical features, such as whether the nasal bone is visible. The presence or absence of the nasal bone adds another data point to the overall risk estimate.
What It Screens For
The primary conditions flagged by increased nuchal translucency are chromosomal abnormalities:
- Down syndrome (trisomy 21), the most common reason the scan is offered
- Edwards syndrome (trisomy 18)
- Patau syndrome (trisomy 13)
- Turner syndrome and other sex chromosome differences
An unusually thick measurement can also signal structural problems unrelated to chromosomes. In studies of pregnancies with increased NT, heart defects, kidney abnormalities, and fluid buildup elsewhere in the body (called hydrops) were among the most common findings. Facial differences like cleft palate and brain abnormalities were also identified in a smaller percentage of cases.
When It’s Done
The scan has a narrow window: 11 weeks to 14 weeks of pregnancy, when the baby measures between roughly 45 and 84 millimeters from crown to rump. Before 11 weeks the baby is too small for an accurate measurement. After 14 weeks the fluid naturally begins to be absorbed, making the reading unreliable. If you miss this window, second-trimester screening options are still available.
What to Expect During the Appointment
The NT scan is a standard abdominal ultrasound. You’ll lie on an exam table while a technician moves a probe over your belly with gel. The whole process typically takes 20 to 30 minutes, though it can run longer if the baby is in a position that makes the neck measurement hard to capture. You may be asked to come with a full bladder, which helps produce a clearer image early in pregnancy. There’s no pain involved and no risk to the baby.
Combined First-Trimester Screening
The NT measurement alone gives useful information, but it becomes significantly more accurate when paired with a blood test. This combination is called first-trimester combined screening. The blood draw checks levels of two proteins produced by the placenta: PAPP-A (pregnancy-associated plasma protein A) and free beta-hCG (a form of human chorionic gonadotropin). Abnormal levels of either protein, combined with the ultrasound measurement and your age, produce a single risk estimate for the major trisomies.
Your age matters in the calculation because the baseline chance of chromosomal conditions rises as maternal age increases. The combined screen folds all of these factors together into a result like “1 in 500” or “1 in 150,” representing the statistical chance that the baby has a given condition.
Understanding Your Results
A normal nuchal translucency measurement generally falls below about 2.5 millimeters, though the exact cutoff depends on the baby’s size at the time of the scan. The 95th percentile reference range runs from about 1.8 to 2.35 mm, meaning most babies measure below that. Measurements above 3.5 mm are associated with the highest likelihood of adverse outcomes.
It’s important to understand that this is a screening test, not a diagnosis. A “high risk” result does not mean the baby definitely has a condition. It means the probability is elevated enough to warrant further testing. Similarly, a “low risk” result significantly reduces the chance of these conditions but cannot eliminate it entirely.
If Your Results Show Elevated Risk
When the NT measurement or combined screening comes back high risk, the next step is usually a diagnostic test that can give a definitive answer. Two options are available:
- Chorionic villus sampling (CVS) can be done as early as 10 to 13 weeks. A small sample of placental tissue is collected and analyzed for chromosomal abnormalities. Initial results (called FISH results) often come back within a few days, with full results in 10 to 14 days.
- Amniocentesis is typically performed after 15 weeks. A thin needle draws a small amount of amniotic fluid, which contains fetal cells for genetic analysis.
Both procedures carry a small risk of miscarriage, so the decision to proceed is personal. Some parents choose cell-free DNA screening (a blood test sometimes called NIPT) as an intermediate step. It’s more accurate than the combined screen but still not diagnostic. If the cell-free DNA result also shows elevated risk, CVS or amniocentesis is the only way to confirm the finding.
For babies with increased NT but normal chromosomes, a detailed anatomy scan and fetal heart scan later in pregnancy can check for structural issues, particularly heart defects.
NT Scan vs. Cell-Free DNA Screening
Cell-free DNA screening has become increasingly popular and has a higher detection rate for the major trisomies. However, the NT scan remains relevant in several situations. It may be recommended when cell-free DNA testing isn’t reliable, such as pregnancies where a twin has been lost early on (vanishing twin), when the mother has had an organ transplant or stem cell therapy, or when insurance doesn’t cover cell-free DNA testing. The NT scan also picks up structural problems like heart defects that cell-free DNA cannot detect, which is why some providers still recommend it even when cell-free DNA is also being used.
The American College of Obstetricians and Gynecologists notes that first-trimester serum screening combined with NT measurement can detect certain genetic deletions, single-gene disorders, and placental abnormalities that cell-free DNA misses. For this reason, the two approaches are complementary rather than interchangeable.