A nuchal translucency (NT) scan is a prenatal ultrasound performed during the first trimester of pregnancy. This screening test helps assess a developing fetus’s likelihood of having certain chromosomal conditions. It is conducted between 11 weeks and 13 weeks and 6 days of gestation, or when the fetus measures between 45 and 84 millimeters from crown to rump.
What the NT Scan Measures
The NT scan specifically measures the fluid-filled space at the back of the fetal neck. This measurement, known as nuchal translucency, is obtained using an ultrasound transducer. All fetuses have a small amount of fluid in this area.
An increased thickness of this fluid can be associated with a higher probability of chromosomal abnormalities, such as Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), and Trisomy 13 (Patau syndrome). A larger NT measurement may also suggest an increased risk for congenital heart conditions or other structural irregularities.
Understanding NT Scan Accuracy
The NT scan is a screening tool, meaning it estimates the probability of a condition rather than providing a definitive diagnosis. When used alone, the NT scan can detect approximately 70% to 80% of Down syndrome cases.
The accuracy of the NT scan improves when combined with maternal blood tests, which measure specific hormone levels like PAPP-A and free beta-hCG. This combined first-trimester screening can achieve a detection rate of 83% to 92% for Down syndrome. Some integrated screening approaches, which combine NT with other markers, can reach detection rates of 94% to 96%.
This combined approach has a false positive rate of about 5%, meaning that around 5% of those screened will receive a “high-risk” result even if the fetus does not have a chromosomal abnormality. Results are usually presented as a risk ratio, such as 1 in 300, indicating the statistical likelihood of a condition. A high-risk screening result does not confirm a diagnosis, but rather suggests a need for further investigation.
Factors Influencing Accuracy
The reliability of an NT scan is influenced by several factors. The gestational age of the fetus is important; the scan must be performed within the precise window of 11 weeks 0 days to 13 weeks 6 days. Beyond this timeframe, the fluid behind the neck begins to reabsorb, making accurate measurement difficult and rendering the results less reliable.
The skill and experience of the sonographer performing the ultrasound also play a role in obtaining accurate measurements. Specialized training and adherence to specific protocols are important for consistent results. The quality and calibration of the ultrasound equipment can likewise affect measurement precision, as even small variations in millimeter readings can alter the calculated risk.
Maternal factors, such as maternal age, are integrated into the risk assessment calculation. While maternal age increases the baseline risk for certain chromosomal conditions, the NT scan and combined screening provide a more refined, individualized risk estimate.
Interpreting Results and Next Steps
After an NT scan, results are categorized as either low risk or high risk. A low-risk result is reassuring, indicating a reduced probability of the screened conditions. However, it does not eliminate all possibility of a chromosomal abnormality or other issues, as no screening test is 100% accurate.
A high-risk result means there is an increased statistical likelihood of a chromosomal abnormality, but it is not a diagnosis. In such cases, healthcare providers will discuss options for further evaluation. These may include non-invasive prenatal testing (NIPT), which analyzes fetal DNA in the mother’s blood, offering a higher detection rate and lower false positive rate than the combined screen.
For a definitive diagnosis, invasive procedures like chorionic villus sampling (CVS) or amniocentesis may be recommended. These diagnostic tests analyze fetal cells directly to confirm or rule out a chromosomal condition. If an increased NT measurement is detected, a fetal echocardiogram might also be suggested to further examine the baby’s heart. Discussing results and options with a healthcare provider or genetic counselor is important for personalized guidance.