The nuchal translucency (NT) test is a common ultrasound performed during the first trimester of pregnancy. This non-invasive screening offers an early look at fetal development. It serves as a tool to assess certain aspects of the baby’s health in the womb.
What the Nuchal Translucency Test Assesses
The nuchal translucency measurement examines the fluid-filled space at the back of the baby’s neck. This test is performed between 11 and 14 weeks of pregnancy. The measurement of this fluid can indicate potential risks.
An increased measurement in this area can be associated with a higher likelihood of certain chromosomal conditions. These include Down syndrome (Trisomy 21), Edwards syndrome (Trisomy 18), and Patau syndrome (Trisomy 13). It may also suggest an increased risk for some congenital heart defects.
Why Results May Vary
Several factors can influence the accuracy of the nuchal translucency measurement. The sonographer’s experience and training are important. Even slight differences in how the measurement is taken can impact the outcome, given the fetus’s small size and the scan’s dynamic nature.
The quality and calibration of the ultrasound equipment also contribute to measurement precision. Different ultrasound machine and probe combinations can lead to variability. For instance, a 1 mm error could increase the perceived risk by a factor of five. The baby’s position during the scan can also affect obtaining a clear and accurate measurement, as an optimal view is necessary.
Screening Versus Diagnosis
The nuchal translucency test functions as a screening tool, not a diagnostic one. It identifies pregnancies with an increased likelihood of certain conditions, rather than providing a definitive answer. A normal NT scan indicates a lower chance of a congenital condition, but it does not guarantee the absence of issues.
When performed alone, the NT scan can detect approximately 70% of Down syndrome cases. However, its accuracy significantly improves when combined with maternal blood tests, which measure specific hormone levels like pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (hCG). This combined first-trimester screening can achieve detection rates for Down syndrome ranging from 90% to 96%. False positive results can occur, meaning the test indicates a higher risk when the baby is healthy, which can cause unnecessary worry.
Next Steps After Your Scan
If the nuchal translucency measurement is elevated or indicates an increased risk, further discussions with a healthcare provider or genetic counselor are recommended. These professionals can help interpret the results and explain potential implications. They will also outline options for additional evaluation.
Diagnostic tests, such as chorionic villus sampling (CVS) or amniocentesis, may be recommended for a definitive diagnosis. CVS occurs earlier in pregnancy, between 10 and 13 weeks, involving a small tissue sample from the placenta for genetic analysis. Amniocentesis is performed later, around 15 to 18 weeks, collecting a sample of the amniotic fluid. Both procedures carry a small risk of miscarriage, ranging from 0.25% to 1.0%. Even with an increased NT measurement, many babies are born healthy, and these further tests offer more certainty about the baby’s chromosomal status.