The Nuchal Translucency (NT) scan is a specialized ultrasound performed during the first trimester of pregnancy. This screening test occurs between 11 weeks and 13 weeks and six days of gestation, or when the fetus measures 45 to 84 millimeters from crown to rump. The purpose is to measure the small, fluid-filled space beneath the skin at the back of the baby’s neck, known as the nuchal fold. The NT scan is not painful, as it uses external contact, similar to any standard ultrasound.
How the Nuchal Translucency Scan is Performed
The Nuchal Translucency scan begins like most ultrasound examinations, with the pregnant person lying on an examination table. The procedure uses a handheld device called a transducer to send sound waves into the body. These waves bounce off internal structures and return to the transducer, which the machine then translates into a real-time image on a monitor.
To ensure the sound waves travel effectively and create clear images, a layer of water-soluble gel is applied to the lower abdomen. The sonographer then moves the transducer over the gelled area, carefully manipulating it to get the ideal view of the developing fetus. The sonographer must obtain a specific sagittal view, showing the baby in a neutral position, to accurately measure the thickness of the translucent space behind the neck.
In most cases, the abdominal approach provides sufficient image quality to complete the necessary measurements of the nuchal fluid. However, if the baby’s position or other factors prevent a clear visualization, a transvaginal ultrasound may be required. This internal approach involves gently inserting a smaller, lubricated transducer into the vagina to get closer to the uterus. While this may feel slightly different than the abdominal scan, it is also typically not painful, though it can cause a feeling of pressure.
Preparing for the Scan and Common Sensations
While the scan itself is not painful, preparation and procedural sensations are the most common sources of discomfort. Patients are often asked to arrive with a full bladder, which helps push the uterus higher for better visualization during the early first trimester. Maintaining a full bladder for an extended period, sometimes after drinking water an hour before the appointment, can lead to temporary pressure and an uncomfortable urge to urinate.
Once the scan begins, the first sensation is the coolness of the ultrasound gel as it is spread across the abdomen. Following this, the sonographer will begin to apply pressure with the transducer, moving it across the skin to capture the images needed for measurement. This pressure is necessary to ensure proper contact and to maneuver the baby slightly if their position is not optimal for measuring the nuchal fluid.
The pressure should not cause sharp pain, but it is often firm, particularly when the sonographer is trying to get a clear image of a small structure. The entire examination can take an estimated 30 to 45 minutes to complete. Patients should communicate any severe or lasting discomfort to the sonographer, though minor pressure is a normal part of the process.
What the NT Measurement Reveals
The primary goal of the Nuchal Translucency scan is to provide a risk assessment for certain genetic conditions. A higher-than-average measurement of the fluid in the nuchal fold can indicate an increased statistical risk for chromosomal anomalies. These include Trisomy 21 (Down syndrome), Trisomy 18 (Edwards’ syndrome), and Trisomy 13 (Patau’s syndrome).
The NT scan is a screening tool only, meaning it determines the likelihood of a condition, not a definitive diagnosis. A greater NT measurement may also suggest an elevated risk for congenital heart defects or other structural issues. The final risk percentage is calculated by combining the NT measurement with blood test results that analyze markers, such as free beta-human chorionic gonadotropin (hCG) and Pregnancy-Associated Plasma Protein-A (PAPP-A).
This combination of the ultrasound measurement and the blood work is known as the combined first-trimester screening. This assessment provides a more accurate risk figure than either test could offer individually. If the calculated risk is elevated, further diagnostic testing options, such as chorionic villus sampling (CVS) or amniocentesis, may be offered.