What is the normal value for NTD risk?

In clinical practice, many pregnant women undergo various prenatal tests, among which the Down syndrome screening (commonly referred to as “Tang screening”) is a frequently chosen option. However, during testing, certain abnormalities or uncertainties in test results and data may arise, often triggering anxiety among expectant mothers. So, what constitutes a normal value for neural tube defect (NTD) risk?

What is the normal value for NTD risk?

A nuchal translucency (NT) measurement of less than 2.5 mm is considered normal. NT refers to the thickness of the fluid-filled space at the back of the fetal neck; values exceeding 2.5 mm fall outside the normal range. The NT scan is typically performed between 11 weeks and 13 weeks + 6 days of gestation, using B-mode ultrasound to measure the thickness of this nuchal translucency. Generally, a thinner NT measurement is preferable. It is not standard practice to categorize NT findings as “high-risk” or “low-risk”; instead, clinicians describe the result based on NT thickness. An increased NT thickness warrants further diagnostic evaluation.

Down syndrome screening includes risk assessment for trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and neural tube defects (NTDs). “NTD” specifically refers to neural tube defect screening. This screening is primarily used to assess the likelihood of fetal structural anomalies. A low-risk result indicates a very low probability of fetal anomalies, though it does not completely rule out the possibility.

A high-risk result only signifies an elevated probability of anomalies—not a definitive diagnosis or 100% certainty of occurrence. If the screening indicates higher risk, confirmatory diagnostic testing—such as amniocentesis—may be recommended. We hope this information has been helpful!