The Nuchal Translucency (NT) scan is a widely utilized first-trimester screening tool. This non-invasive ultrasound provides an early assessment of the developing fetus and helps determine the statistical likelihood of certain genetic conditions. Understanding the results, particularly what constitutes a typical measurement, is often a source of anxiety. The NT scan is only one part of a broader screening process used to determine the need for further, more definitive testing.
Understanding the Nuchal Translucency Screening
The Nuchal Translucency scan is a specialized ultrasound that measures a fluid-filled space at the back of the fetal neck, known as the nuchal fold. This fluid layer is present in all fetuses during early development. Measuring its thickness helps healthcare providers estimate the probability of a chromosomal abnormality, such as Down syndrome (Trisomy 21), Edwards syndrome (Trisomy 18), or Patau syndrome (Trisomy 13).
The timing of this scan is important because the fluid is naturally reabsorbed later in the pregnancy. For an accurate measurement, the NT scan must be performed between 11 weeks and 13 weeks and six days, or when the fetus measures 45 to 84 millimeters from crown to rump.
The NT scan is often combined with a maternal blood test to form the Combined First-Trimester Screening. This blood test measures the levels of two substances: pregnancy-associated plasma protein-A (PAPP-A) and free beta-human chorionic gonadotropin (hCG). Combining the ultrasound measurement with these biochemical markers and the mother’s age increases the overall accuracy of the risk assessment.
Defining the Normal Measurement Range at 13 Weeks
The thickness of the nuchal translucency is directly related to the fetus’s gestational age, meaning the measurement increases as the fetus grows. For a fetus at 13 weeks gestation, a normal NT measurement is considered to be under 2.8 millimeters (mm).
Some clinical protocols may use a slightly different cutoff, occasionally setting the threshold for increased risk at 3.0 mm or 3.5 mm. However, the measurement is always compared against the expected range for the precise crown-rump length of the fetus.
The NT measurement is only one factor used to calculate statistical risk. A result within the established normal range suggests a low statistical probability of a chromosomal condition. Conversely, a higher measurement, such as one greater than 3.5 mm, correlates with a higher statistical risk of a chromosomal or structural issue.
The NT scan is a screening test, not a diagnostic one. A high measurement merely indicates an increased likelihood that warrants further investigation. Many fetuses with an increased NT measurement, even up to 3.5 mm, are chromosomally and structurally normal.
Next Steps Following an Abnormal Screening Result
If the combined first-trimester screening indicates a high-risk result, the statistical chance of a chromosomal abnormality is elevated. This outcome does not provide a definitive diagnosis but suggests the need for further, conclusive testing options. These next steps are designed to move from a risk assessment to a clear diagnosis or to rule out the conditions in question.
One common follow-up option is Non-Invasive Prenatal Testing (NIPT), which analyzes cell-free fetal DNA found in the mother’s bloodstream. NIPT is a highly accurate screening test for Trisomy 21, 18, and 13. It is often recommended as the first step after an elevated NT result because it carries no risk of miscarriage. If NIPT is positive, or if a definitive answer is required, diagnostic procedures are available.
The two main diagnostic tests are Chorionic Villus Sampling (CVS) and Amniocentesis. CVS involves sampling placental tissue and is performed between 10 and 13 weeks. Amniocentesis involves sampling amniotic fluid and is usually performed after 15 weeks of gestation.
These invasive procedures analyze the fetal chromosomes directly, providing a clear diagnosis, but they carry a small risk of complications, including miscarriage. For any increased NT measurement, a detailed fetal echocardiogram may also be recommended around 20 weeks, as increased fluid space can be associated with a higher risk of congenital heart defects.