A nuchal translucency ultrasound is a prenatal screening that measures the small pocket of fluid at the back of a developing baby’s neck during the first trimester. The size of this fluid collection helps estimate the baby’s risk for certain genetic conditions, including Down syndrome, and congenital heart defects. It’s one of the earliest screening tools available in pregnancy, typically performed between 11 and 13 weeks.
What the Scan Measures
During the ultrasound, a sonographer measures the thickness of the nuchal fold, a layer of tissue at the back of the baby’s neck. All babies have some fluid in this area, but a larger-than-expected measurement can signal a higher risk of chromosomal conditions or structural problems. The scan specifically screens for Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), Patau syndrome (trisomy 13), and congenital heart defects.
It’s important to understand what “screening” means here. This ultrasound doesn’t diagnose anything. It calculates a probability. A thicker nuchal fold raises the statistical risk, but many babies with slightly increased measurements turn out to be perfectly healthy. Conversely, a normal measurement reduces risk but doesn’t eliminate it entirely.
When It Needs to Happen
The timing window is narrow. The scan must be done between 11 weeks and 13 weeks and 6 days of pregnancy, and the baby needs to measure between 45 and 84 millimeters from crown to rump. Before 11 weeks, the baby is too small for an accurate measurement. After nearly 14 weeks, the fluid tends to be reabsorbed, making the results unreliable. If you miss this window, your provider will discuss alternative screening options.
What to Expect During the Appointment
The scan is a standard abdominal ultrasound. You’ll likely be asked to drink about 24 ounces of clear fluid at least an hour beforehand and avoid emptying your bladder until after the exam. A full bladder pushes the uterus into a better position for imaging. The sonographer applies gel to your abdomen and uses a transducer to capture images of the baby, then takes precise measurements of the fluid space behind the neck. The process is painless and typically takes 20 to 30 minutes, though it can run longer if the baby isn’t in an ideal position.
Combined First Trimester Screening
The nuchal translucency ultrasound is most accurate when paired with a blood test, and most providers offer them together as a “combined first trimester screening.” The blood draw measures two proteins produced by the placenta. Your provider’s lab combines these protein levels with the ultrasound measurement, your age, and other factors to generate an overall risk score.
This combined approach detects approximately 96% of pregnancies affected by Down syndrome, with even higher detection rates for trisomy 13 and trisomy 18. By comparison, the ultrasound measurement alone is less precise. The combined screening is the reason your provider will often schedule a blood draw around the same visit as the scan.
Understanding Your Results
Results come back as a ratio, something like “1 in 5,000” or “1 in 150.” A lower number in the denominator means higher risk. There’s no single cutoff that divides “normal” from “abnormal,” but your provider will explain where your result falls relative to a threshold your lab uses to flag increased risk. Most people receive a low-risk result and need no further testing related to these conditions.
If your result shows increased risk, that does not mean your baby has a genetic condition. It means additional testing is worth considering. Many high-risk screening results turn out to be false positives after further evaluation.
What Happens After a High-Risk Result
A high-risk screening result opens the door to several next steps, and you’ll have choices. Your provider should offer genetic counseling to help you understand what the numbers actually mean for your pregnancy. From there, you can pursue diagnostic testing that gives a definitive yes-or-no answer.
The two main diagnostic options are chorionic villus sampling (CVS), which can be done between 10 and 13 weeks, and amniocentesis, typically performed after 15 weeks. Both involve collecting a small sample of cells for chromosomal analysis. These tests carry a small risk of complications, which a genetic counselor will walk you through in detail. Some people also opt for cell-free DNA screening, a blood test that analyzes fragments of the baby’s DNA circulating in the mother’s blood. It’s more accurate than combined screening but still not diagnostic.
If an enlarged nuchal fold or any structural concern is identified on the scan, current guidelines also recommend a detailed follow-up ultrasound between 18 and 22 weeks to check for physical abnormalities that may not be visible earlier in pregnancy.
Who Should Get This Screening
Current guidance from the American College of Obstetricians and Gynecologists recommends that all pregnant patients be offered prenatal genetic screening and diagnostic testing regardless of age or baseline risk. The old practice of reserving screening for people over 35 has been replaced by a more inclusive approach. Whether you actually pursue the screening is a personal decision, but it should be presented as an option at an early prenatal visit so you have time to schedule it within the narrow window.