Average Nuchal Translucency Measurement

Nuchal translucency (NT) refers to the clear tissue space at the back of a developing fetus’s neck. The NT is measured during ultrasonography performed between gestational weeks 11-14. Once the technician has found the nuchal fold, they use a caliper tool to measure the the NT as it appears on the screen. A large amount of space usually indicates a genetic abnormality, such as Down syndrome, trisomy, or heart defect.

Average NT Measurement in a Healthy Fetus

• A study sponsored by the National Institute for Child Health and Development determined that “The mean NT for the normal fetuses was 1.5 mm.” Average thicknesses at 12-13 weeks of gestation were 1.7 mm in the 50th percentile and 2.8 mm in the 95th percentile.

Average NT Measurement Indicating Abnormality or Down Syndrome

• Any NT measurement between 2.5 and 3.0 mm may indicate a genetic problem and require further testing. A measurement greater than 3.0 mm is considered abnormal. A 2010 study by the Wolfson Institute of Preventive Medicine published in the "Journal of Medical Screening" found the average NT measurement in fetuses with Down syndrome to be 4.9 mm.

NT Measurements and Congenital Heart Defects

• NT measurements can also be used to screen for potential congenital heart defects. A study by the Fetal Medicine Foundation found that 55% to 56% of cases with a NT measurement above the 95th percentile had “major defects of the heart and great arteries.” In this study, the 95th to 99th percentiles were 2.2 mm to 3.5 mm, respectively.