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Nuchal Translucency Calculator

The nuchal translucency (NT) measurement—the thickness of fluid in the back of the fetal neck—is a key marker in first-trimester prenatal screening. Taken between 11 and 14 weeks, an elevated NT can suggest increased risk of chromosomal abnormalities including Down syndrome, Edwards syndrome, and Patau syndrome. This calculator interprets your NT measurement against established percentile charts and helps contextualize findings alongside crown-rump length and gestational age estimates.

Last updated: April 16, 2026

Creators Małgorzata Koperska, MD

Reviewers Łucja Zaborowska and Adena Benn

465 people find this calculator helpful

Understanding the Nuchal Translucency Test

The nuchal translucency measurement is obtained during an ultrasound examination in the first trimester, typically between 11 and 14 weeks of gestation. The measurement represents the depth of a fluid collection beneath the skin at the posterior neck of the fetus.

During the scan, a sonographer will:

Measure the NT thickness and compare it against age-specific reference ranges
Determine the crown-rump length (CRL), the distance from the top of the head to the base of the buttocks
Estimate gestational age based on fetal anatomy
Screen for other structural or anatomical markers

NT screening does not diagnose genetic disorders—it identifies pregnancies at higher statistical risk. Interpretation requires integration with maternal age, blood chemistry results, and family history for comprehensive risk assessment.

Calculating Expected Nuchal Translucency

The expected NT thickness for a given fetal size follows a linear relationship with crown-rump length. The formula below predicts the typical NT value; actual measurements are then plotted against population percentiles to determine whether the result falls within normal limits.

Expected NT (mm) = 0.437 + (0.01969 × CRL)

Gestational age (days) = round(8.052 × √(CRL × 1.037) + 23.73)

NT — Nuchal translucency thickness, measured in millimeters
CRL — Crown-rump length of the fetus, measured in millimeters
Gestational age — Age of the pregnancy estimated from fetal biometry, expressed in days

Chromosomal Abnormalities and Elevated Nuchal Translucency

An increased NT thickness is associated with higher risk of several genetic conditions:

Down syndrome (Trisomy 21): The most common chromosomal abnormality compatible with life. Infants often have characteristic facial features, congenital heart defects, and intellectual disability ranging from mild to moderate.
Edwards syndrome (Trisomy 18): A severe condition with multiple organ involvement. Most affected infants do not survive beyond the first year of life.
Patau syndrome (Trisomy 13): Characterized by severe developmental abnormalities. Survival beyond infancy is uncommon.
Congenital heart defects: Even without chromosomal abnormalities, elevated NT can indicate structural cardiac malformations requiring postnatal intervention.

Measurements below the 95th percentile are considered within normal limits. Risk increases substantially above this threshold, particularly when NT exceeds 3.5 mm.

Important Considerations for NT Interpretation

Several factors influence how NT results should be understood and acted upon:

Normal NT does not exclude genetic disorders — A normal NT measurement significantly reduces but does not eliminate the risk of chromosomal abnormalities. Additional screening methods—including maternal serum biomarkers (PAPP-A, beta-hCG), cell-free fetal DNA testing, and detailed anatomical survey—provide complementary risk stratification.
Timing matters for accurate measurement — NT screening is valid only between 11 weeks 2 days and 14 weeks 1 day of gestation. Measurements obtained outside this window may not be reliably compared to published reference ranges, potentially leading to misinterpretation.
Technical quality affects reliability — Obtaining an accurate NT measurement requires proper fetal positioning, image orientation, and operator expertise. Poor image quality or inadequate measurement technique can produce falsely elevated or low values that do not reflect true risk.
Maternal age influences overall risk — Older maternal age increases the baseline risk of chromosomal abnormalities independent of NT thickness. A 35-year-old with identical NT to a 25-year-old faces higher overall risk due to age-related increases in aneuploidy.

Interpreting Results: Risk Stratification

The NT percentile determines how your measurement compares to other fetuses of the same crown-rump length:

Below 95th percentile: Approximately 97% of babies born healthy; genetic defect risk ~0.2%
95th to 99th percentile: 93% born healthy; genetic defect risk increases to ~3.7%
3.5–4.4 mm: Healthy outcome ~70%; genetic defect risk ~21%
4.5–5.4 mm: Healthy outcome ~50%; genetic defect risk ~33%
≥5.5 mm: Significantly elevated risk; further diagnostic testing strongly recommended

These figures emphasize that even moderately elevated NT does not indicate certain abnormality; many pregnancies with high NT values proceed to deliver unaffected babies. Conversely, a normal NT provides reassurance but does not guarantee a healthy outcome.

Frequently Asked Questions

What measurement is considered normal nuchal translucency at 12 weeks gestation?

At 12 weeks, a normal NT measurement is typically 1.9 mm or less, though the exact upper limit varies slightly depending on the reference population and measurement criteria used. Fetuses with NT at or below the 95th percentile are considered low-risk. If your measurement exceeds this threshold, your sonographer may recommend cell-free fetal DNA testing, maternal serum screening, or repeat ultrasound to refine risk assessment.

How is gestational age estimated from the nuchal translucency scan?

Gestational age is not calculated from the NT itself, but rather from the crown-rump length, which is routinely measured during the same ultrasound. Using the CRL measurement in the formula 8.052 × √(CRL × 1.037) + 23.73 yields gestational age in days. This method is most accurate in the first and second trimester; accuracy decreases in later pregnancy when skeletal and organ development becomes more variable.

Can a normal NT scan completely rule out Down syndrome?

No. While a normal NT significantly reduces risk, it does not exclude Down syndrome or other chromosomal abnormalities. Approximately 20% of affected pregnancies have normal NT measurements. To achieve comprehensive risk assessment, combine NT results with first-trimester biochemistry (PAPP-A and beta-hCG levels), cell-free fetal DNA testing, and family history. Integrated screening approaches detect 95% or more of Down syndrome cases.

What does an NT measurement of 3.0 mm at 13 weeks indicate?

An NT of 3.0 mm at 13 weeks is elevated above the 95th percentile and warrants further investigation. While many pregnancies with this finding result in healthy births, the risk of chromosomal abnormality is materially increased. Recommend discussion with maternal-fetal medicine for detailed anatomical survey, consideration of cell-free fetal DNA testing (non-invasive, low-risk), and possibly amniocentesis if diagnostic certainty is desired.

Does maternal age affect the interpretation of nuchal translucency results?

Absolutely. Maternal age is an independent risk factor for chromosomal abnormalities. A 40-year-old with an identical NT to a 25-year-old carries substantially higher baseline risk due to age-related increases in egg aneuploidy. Risk calculators integrate maternal age, NT measurement, and biomarker levels to generate individualized risk estimates rather than relying on NT alone.

Why would NT appear thicker in one pregnancy than another at the same gestational age?

Biological variation is normal; not all fetuses at the same gestational age have identical NT thickness. Genetic factors, fluid status, cardiac function, and other physiological variables influence NT measurement. This is why reference percentiles are essential—they account for normal biological range. Additionally, measurement technique, image quality, and operator experience can introduce variation, emphasizing the importance of obtaining scans at specialized prenatal imaging centers.

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