What Is the Nuchal Translucency Scan?

The nuchal translucency (NT) scan is a specialized ultrasound performed between 11 weeks and 13 weeks, 6 days of pregnancy. It measures a specific pocket of fluid at the back of the baby's neck. All babies have some fluid there, but an increased measurement can be associated with a higher likelihood of certain chromosomal conditions, particularly Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13). It can also be linked with some heart defects.

The NT measurement is one piece of a larger screening picture. On its own, it does not diagnose anything. It contributes to a risk assessment that helps you and your provider decide whether further testing is appropriate.

How the Scan Works

The NT scan is performed abdominally in most cases, though occasionally a transvaginal approach is used if the baby's position or maternal body composition makes abdominal imaging difficult. The timing window is narrow: the measurement is only reliable between 11 and 13+6 weeks, and the baby's crown-rump length needs to fall between 45 mm and 84 mm for the result to be valid.

During the scan, the sonographer will position the image to get a clear sagittal (side-profile) view of the baby and measure the thickest part of the translucent space at the nape of the neck. The measurement is recorded in millimeters, and even fractions of a millimeter matter. This is a scan that requires a trained, credentialed sonographer, so it is typically performed at a specialist center or by a provider certified in NT measurement.

At the same time, the sonographer will confirm gestational age, check the heartbeat, and often look for the presence of the nasal bone, which is another marker used in the overall assessment.

Combined First Trimester Screening

The NT scan is most informative when combined with a blood test drawn between 9 and 13 weeks. This combination is called combined first trimester screening. The blood work measures two substances:

  • PAPP-A (Pregnancy-Associated Plasma Protein-A): Lower-than-expected levels can be associated with an increased risk of chromosomal conditions and, independently, with placental complications later in pregnancy.
  • Free beta-hCG (Human Chorionic Gonadotropin): Higher-than-expected levels of this pregnancy hormone can also be associated with increased chromosomal risk.

Your provider or genetic counselor takes the NT measurement, the blood results, and your age (which correlates with background risk for chromosomal conditions) and runs them through an algorithm that produces a personalized risk ratio.

Understanding Your Results: Risk Ratios, Not Diagnoses

This distinction is the single most important thing to understand about first trimester screening. The result is a probability, not a diagnosis.

A result might read something like "1 in 5,000 risk for Down syndrome" or "1 in 150 risk for Edwards syndrome." What that means is that among a large group of pregnancies with similar measurements and blood values, that proportion would be expected to have the condition. It does not tell you whether your specific baby is affected.

Most screening programs use a cutoff to define "screen positive" and "screen negative." A common cutoff is 1 in 300. A result of 1 in 100 would be screen positive; a result of 1 in 1,000 would be screen negative. But these labels can be misleading. A "screen positive" result does not mean your baby has a chromosomal condition. It means the screening suggests a higher-than-average probability, and diagnostic testing is available if you want more information.

The combined first trimester screening detects approximately 85 to 90 percent of Down syndrome cases, with a false positive rate of about 5 percent. That means roughly 1 in 20 people who screen positive will receive further testing that confirms the baby does not have the condition.

Screening vs. Diagnostic Testing

This is a critical distinction, and it is worth spending time with.

Screening Tests

Screening tests (including the NT scan, combined first trimester screening, and cell-free DNA/NIPT) estimate risk. They are non-invasive and carry no risk to the pregnancy. They can tell you whether further testing might be worth considering, but they cannot confirm or rule out a condition.

Diagnostic Tests

Diagnostic tests provide a definitive answer. The two diagnostic options in pregnancy are:

  • Chorionic villus sampling (CVS): Performed between 10 and 13 weeks, CVS collects a small sample of placental tissue. Results are typically available within 1 to 2 weeks.
  • Amniocentesis: Performed after 15 weeks, amniocentesis collects a sample of amniotic fluid. Results usually take 1 to 3 weeks.

Both procedures carry a small risk of miscarriage, generally cited as roughly 1 in 300 to 1 in 500 when performed by an experienced specialist. The risk has decreased significantly in recent years as techniques have improved.

Making Informed Decisions

Whether to pursue screening, and what to do with the results, is a deeply personal decision. There is no single right answer. Some families want as much information as possible as early as possible. Others prefer to decline screening entirely and meet their baby without that data. Both paths are valid.

If you do receive a screen-positive result, consider the following before deciding on next steps:

  • Talk to a genetic counselor. Genetic counselors are specifically trained to help you understand risk ratios in context, walk through the options for diagnostic testing, and support whatever decision you make. Their role is informational and non-directive.
  • Remember the false positive rate. A screen-positive result is far more likely to be a false positive than a true positive, especially if your background risk is low.
  • Take the time you need. Screening results can feel urgent, but in most cases, there is time to process the information, ask questions, and decide on your own timeline.

Some families who receive diagnostic confirmation of a chromosomal condition choose to continue the pregnancy with the benefit of preparation and planning. Others make different decisions. The value of screening is that it opens the door to information, and what you do with that information is yours to decide.

A Note on Cell-Free DNA Testing (NIPT)

Cell-free DNA screening (also called non-invasive prenatal testing, or NIPT) is a blood test available from about 10 weeks onward. It screens for the most common trisomies and can also report fetal sex. NIPT has a higher detection rate and lower false positive rate than combined first trimester screening for the conditions it covers. However, it is still a screening test, not a diagnostic one. Positive NIPT results should always be confirmed with CVS or amniocentesis before any clinical decisions are made.

Your provider may offer NIPT as an alternative to or in addition to the NT scan and combined screening. The decision depends on your preferences, your risk profile, and your insurance coverage.

Find Guidance Through BAABY's Provider Directory

Navigating screening decisions can feel complex, especially during your first pregnancy. BAABY's provider directory can help you find OB-GYNs, midwives, and genetic counselors in your area who are experienced in prenatal screening and who will take the time to answer your questions. The right provider will respect your autonomy and support whatever path you choose.