The Nuchal Translucency (NT) scan is a non-invasive ultrasound procedure offered during the first trimester of pregnancy. This screening tool provides an early assessment of the fetus’s statistical risk for certain congenital conditions. The scan is performed within a narrow window, typically between 11 weeks and 13 weeks and six days of gestation, to ensure accurate measurement. The NT measurement is often combined with maternal blood test results to give a comprehensive first-trimester screening result.
What the Nuchal Translucency Scan Measures
The NT measurement assesses a naturally occurring, fluid-filled space located beneath the skin at the back of the fetal neck. This fluid collection is a normal part of early development. Sonographers measure the maximum thickness of this translucent area using high-resolution ultrasound imaging.
The specific timing of the scan is determined by the fetus’s size, not the calendar week of pregnancy. For the NT measurement to be valid, the Crown-Rump Length (CRL) must be between 45 millimeters and 84 millimeters. The 12-week mark falls within this optimal measurement window. The thickness of this fluid-filled space is particularly informative due to the development of the fetal lymphatic system and blood flow during this period.
Defining the Normal Range at 12 Weeks
The NT measurement changes as the fetus grows, so the definition of a “normal” thickness depends on the precise gestational age or Crown-Rump Length. At approximately 12 weeks, the upper limit for a low-risk measurement is less than 3.5 millimeters. More specifically, the median NT thickness at 12 weeks is often around 1.2 to 1.4 millimeters. The 95th percentile, the point above which only 5% of fetuses measure, falls around 2.5 millimeters.
This measurement is not interpreted in isolation. The combined first-trimester screen integrates the NT value with maternal blood work, which measures levels of free beta-human chorionic gonadotropin (free β-hCG) and pregnancy-associated plasma protein-A (PAPP-A). This combination provides a refined, calculated risk factor for chromosomal conditions. Results are reported as a statistical probability, such as a 1 in 10,000 chance, rather than a simple pass or fail.
Understanding Elevated Nuchal Translucency Readings
An NT measurement exceeding the 95th percentile for that gestational age (greater than 2.5 millimeters at 12 weeks) signifies an increased statistical risk, not a definitive diagnosis. This elevated thickness is associated with a higher chance of chromosomal abnormalities. The most commonly associated conditions are Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), and Trisomy 13 (Patau syndrome).
An increased NT measurement is also a potential indicator of structural anomalies, particularly defects affecting the fetal heart. Cardiovascular anomalies are the most frequently encountered defects in chromosomally normal fetuses with an increased NT. The risk of an adverse outcome increases progressively as the NT thickness increases, even in cases where the fetal chromosomes are ultimately found to be typical. It is important to remember that many fetuses with an increased NT will still be born perfectly healthy, especially if the measurement is only mildly elevated.
Next Steps Following a Screening Result
When a screening result indicates an increased risk, a discussion with a genetic counselor or a maternal-fetal medicine specialist is the typical next step. The first option presented is Non-Invasive Prenatal Testing (NIPT), which analyzes fetal DNA circulating in the mother’s blood. NIPT is highly accurate for screening for Trisomy 21, 18, and 13, offering a refined risk assessment without posing risk to the pregnancy.
If the NIPT result suggests a high risk, or if the NT is significantly elevated (3.5 millimeters or more), definitive diagnostic testing may be offered. These invasive procedures, Chorionic Villus Sampling (CVS) or Amniocentesis, analyze fetal cells to confirm or rule out a chromosomal condition with near-certainty. CVS is performed earlier (10 to 13 weeks), while Amniocentesis is performed later (typically after 15 weeks). Due to the link to structural issues, a detailed fetal echocardiogram to evaluate the heart may also be recommended later in the pregnancy.