The Nuchal Translucency (NT) scan is a prenatal screening tool offered during the first trimester of pregnancy. This non-invasive ultrasound assesses a fetus’s risk for certain chromosomal conditions, primarily Down syndrome (Trisomy 21), Trisomy 18, and Trisomy 13. As a screening test, its purpose is to calculate a risk probability, not to provide a definitive diagnosis. The accuracy of the NT scan depends on how it is performed and whether its results are combined with other factors.
Defining the Nuchal Translucency Measurement
The NT scan measures the fluid-filled space located at the back of the fetus’s neck. While all fetuses have some fluid here, an increased measurement is associated with a higher likelihood of genetic or structural conditions. The procedure must be performed between 11 weeks and 13 weeks and 6 days of gestation. This timing is important because the fluid begins to be reabsorbed after 14 weeks, making accurate measurement impossible.
The NT measurement is only one part of the risk assessment. To improve accuracy, it is often combined with maternal age and a blood test, forming the Combined First Trimester Screening. This approach analyzes the mother’s blood for two specific markers: Pregnancy-Associated Plasma Protein-A (PAPP-A) and free beta-human chorionic gonadotropin (hCG). The levels of these biochemical markers, along with the NT measurement and maternal age, are statistically analyzed to generate the final risk ratio.
Interpreting NT Scan Reliability
The accuracy of the NT scan is measured by its performance as a screening tool. When used alone, the NT measurement has a detection rate (sensitivity) of approximately 70% to 75% for Down syndrome. This means it correctly identifies three out of every four affected pregnancies as high-risk. The standalone NT scan also has a false positive rate of around 5%, meaning about one in twenty healthy pregnancies is incorrectly flagged as high risk.
The reliability increases significantly when the NT scan is combined with the maternal blood markers (PAPP-A and hCG). This Combined First Trimester Screening typically achieves a detection rate of around 90% for Down syndrome, with a false positive rate remaining low, often around 4% to 5%. The test result is presented as a statistical risk ratio, such as 1 in 500. A low-risk result means the probability of the condition is very low, though it does not guarantee a healthy outcome.
A false negative represents the small percentage of affected pregnancies that the screening test misses. Even with the combined screening’s high detection rate, an affected fetus may still receive a low-risk result. A high-risk result signals an elevated probability, not a definite diagnosis, and typically leads to follow-up testing.
Contextualizing Results with Follow-Up Testing
The NT scan, even when combined with blood tests, remains a screening tool that only estimates risk. A high-risk result does not confirm a diagnosis, but rather indicates that further, more definitive testing should be considered. This next step involves diagnostic tests, which provide a clear yes or no answer regarding the presence of a chromosomal condition.
Non-Invasive Prenatal Testing (NIPT)
One common first step after an elevated risk is Non-Invasive Prenatal Testing (NIPT), also known as cell-free DNA (cfDNA) testing. This blood test analyzes fragments of the fetus’s DNA circulating in the mother’s bloodstream to screen for chromosomal abnormalities.
Invasive Diagnostic Procedures
If a definitive diagnosis is required, the options involve invasive procedures: Chorionic Villus Sampling (CVS) or Amniocentesis. CVS is typically performed earlier in pregnancy and involves taking a tissue sample from the placenta. Amniocentesis is usually performed later and involves sampling the amniotic fluid surrounding the fetus. These diagnostic procedures carry a small risk of complication but offer a confirmed genetic result.