The nuchal translucency (NT) scan is a prenatal screening tool used in early pregnancy. This ultrasound assesses the risk of certain chromosomal conditions, like Down syndrome, and some heart defects. The scan provides information that guides further prenatal care decisions.
Understanding the Nuchal Translucency Scan
The nuchal translucency scan measures the fluid-filled space, known as the nuchal fold, at the back of a baby’s neck. All fetuses have some fluid, but an increased amount can indicate certain conditions. This non-invasive procedure is performed between 11 weeks and 13 weeks and 6 days of pregnancy. This timing is important because the fluid behind the neck is reabsorbed after 14 weeks, making accurate measurement difficult.
During the scan, a sonographer applies gel to the abdomen and uses a handheld transducer to image the fetus. The fluid area behind the neck is measured in millimeters. If abdominal imaging is unclear, a transvaginal ultrasound may be used to obtain better views. The scan usually takes about 30 minutes.
Interpreting Nuchal Translucency Results
The measurements obtained from the NT scan are used to generate a risk assessment, not a definitive diagnosis. This assessment considers the NT measurement, maternal age, and often blood test results if part of a combined screening. A higher NT measurement increases the chance of a chromosomal or genetic condition. An NT measurement of 3.5 mm or more indicates an increased chance of such conditions.
A “high risk” result indicates a greater statistical chance of a condition, but does not confirm a diagnosis. Conversely, a “low risk” result does not guarantee the absence of a condition. The NT scan functions solely as a screening tool, identifying pregnancies that may benefit from further investigation. Many babies with increased NT measurements are born healthy.
Key Factors Affecting Accuracy
Several variables can influence the reliability of an NT scan. The sonographer’s skill and experience are important. Certified sonographers, such as those by the Fetal Medicine Foundation (FMF), are more likely to obtain accurate measurements. The quality and calibration of the ultrasound equipment also play a role.
The gestational age of the fetus is another important factor. Measurements are most accurate within the optimal window of 11 to 14 weeks. Performing the scan too early or too late can lead to less reliable results. Fetal positioning during the scan also impacts accuracy, requiring a neutral neck position for proper measurement.
Understanding Nuchal Translucency Accuracy Rates
NT scan accuracy varies depending on whether it is used alone or combined with other screening methods. As a standalone assessment, the NT scan has an accuracy rate of approximately 70-75% for detecting chromosomal abnormalities like Down syndrome (Trisomy 21). This means it may miss about 30% of cases. A higher NT measurement also indicates an increased risk for Trisomy 18 (Edwards syndrome), Trisomy 13 (Patau syndrome), and some congenital heart conditions.
Detection rates significantly improve when the NT scan is combined with maternal blood tests measuring hormones like PAPP-A and free beta-hCG. This combined first-trimester screening can achieve detection rates of 83% to 92% for Down syndrome. For Trisomy 18 and 13, detection rates can be higher, reaching around 97% and 92% respectively, at a 5% false-positive rate.
False positive rates occur when the test indicates a higher risk, but the baby is healthy. For combined screening, the false-positive rate is around 5%. False negative rates occur when the test indicates a low risk, but a condition is present. No screening test, including the NT scan, is 100% accurate, and false positives are common with the NT scan alone.
Next Steps After a Nuchal Translucency Scan
If an NT scan indicates a higher risk, further options are discussed. One common next step is Non-Invasive Prenatal Testing (NIPT), a maternal blood test analyzing fetal DNA in the mother’s bloodstream. NIPT is a more accurate screening tool, with detection rates for Down syndrome around 99%. It also has a lower false-positive rate than the combined NT screening.
For a definitive diagnosis, invasive diagnostic tests may be offered. Chorionic Villus Sampling (CVS) involves a tissue sample from the placenta, usually performed between 11 and 14 weeks. Amniocentesis involves sampling amniotic fluid, performed after 15 weeks of pregnancy. These diagnostic tests provide conclusive answers about chromosomal conditions but carry a small risk of miscarriage. A genetic counselor can explain the results and discuss the risks and benefits of further testing.