Nuchal Translucency (NT) Calculator

What is Nuchal Translucency?

Nuchal translucency (NT) is the sonographic appearance of a fluid-filled space at the back of the fetal neck, measured during the first trimester of pregnancy (typically between 11 weeks 0 days and 13 weeks 6 days, corresponding to a CRL of 45–84 mm). This measurement was first described by Nicolaides et al. in the 1990s and has since become a cornerstone of first-trimester screening for chromosomal abnormalities.

All fetuses have some fluid at the back of the neck during the first trimester. An increased NT thickness is associated with a higher risk of chromosomal abnormalities (particularly trisomy 21, 18, and 13), cardiac defects, and other structural anomalies. However, it is important to understand that an increased NT is a screening finding, not a diagnosis — most fetuses with mildly increased NT are chromosomally normal.

NT measurement is most meaningful when expressed as a multiple of the expected median (MoM) for the gestational age, as normal NT thickness increases with CRL. The MoM allows comparison across different gestational ages.

NT Calculation Method

Expected NT (Median)

This regression equation provides the expected median NT for a given CRL. Different models and laboratories may use slightly different coefficients, but this formula is widely used in clinical practice.

NT MoM (Multiple of Median)

An MoM of 1.0 means the measurement is exactly at the median. MoM values above 1.0 indicate measurements larger than expected, and values below 1.0 indicate smaller than expected.

Gestational Age from CRL

This equation estimates gestational age from CRL (Robinson formula). It is most accurate between CRL of 30–84 mm (approximately 8–14 weeks).

95th Percentile NT

Measurements above the 95th percentile are considered increased and warrant further evaluation. An absolute cutoff of 3.0 mm is also commonly used regardless of gestational age.

Interpretation of Results

NT MoM	NT Thickness	Interpretation
< 1.0	Below median	Normal; lower than average risk
1.0–1.5	At/near median	Normal; average risk
1.5–2.0	Above median but < 95th %ile	Likely normal; may warrant monitoring
> 2.0 or > 3.0 mm	Above 95th percentile	Increased risk; further testing recommended
> 3.0 (absolute)	> 3.0 mm at any GA	Significantly increased; diagnostic testing offered
> 3.5 mm	Markedly elevated	High risk for chromosomal and structural anomalies

NT Measurement Diagram

Combined First Trimester Screening

NT measurement alone has a detection rate of approximately 64–70% for Down syndrome at a 5% false-positive rate. When combined with maternal serum markers, the detection rate increases significantly:

• NT alone: ~70% detection rate for trisomy 21
• NT + free beta-hCG + PAPP-A: ~85–90% detection rate (combined first-trimester screening)
• Combined + nasal bone assessment: ~93–95% detection rate
• Cell-free DNA (NIPT): ~99% detection rate (follow-up confirmatory test)

The combined first-trimester screening integrates NT measurement with two maternal blood markers measured between 9–13 weeks:

• PAPP-A (pregnancy-associated plasma protein A): Typically low in trisomy 21 pregnancies (0.4–0.5 MoM)
• Free beta-hCG: Typically elevated in trisomy 21 pregnancies (1.8–2.0 MoM)

These markers are combined with maternal age to generate a patient-specific risk estimate. A risk cutoff of 1:250 or 1:300 at term is commonly used to define "screen-positive" results.

Associated Conditions

Chromosomal Abnormalities

Condition	Typical NT Pattern	Additional Markers
Trisomy 21 (Down syndrome)	Increased NT; present in ~75%	Low PAPP-A, high free beta-hCG, absent nasal bone
Trisomy 18 (Edwards syndrome)	Markedly increased NT	Very low PAPP-A, very low free beta-hCG
Trisomy 13 (Patau syndrome)	Increased NT	Low PAPP-A, variable beta-hCG
Turner syndrome (45,X)	Very large NT / cystic hygroma	Low PAPP-A, high free beta-hCG
Triploidy	Variable	Depends on parental origin

Non-chromosomal Associations

Increased NT in chromosomally normal fetuses may be associated with:

• Congenital heart defects: Present in 4–6% of fetuses with NT > 3.5 mm and normal karyotype
• Diaphragmatic hernia
• Skeletal dysplasias
• Genetic syndromes: Noonan syndrome, Smith-Lemli-Opitz syndrome
• Congenital infections

Most fetuses (approximately 95%) with mildly increased NT (3.0–3.5 mm) and normal chromosomes have normal outcomes. The risk of significant anomalies increases with NT thickness.

NT Measurement Technique

Accurate NT measurement requires strict adherence to the Fetal Medicine Foundation (FMF) protocol:

• CRL range: 45–84 mm (11+0 to 13+6 weeks)
• Fetal position: Midsagittal section with neutral neck position (neither flexed nor extended)
• Magnification: Fetal head and upper thorax should fill at least 75% of the image
• Caliper placement: Inner-to-inner edge measurement at the widest space; calipers placed on the leading edges of the NT lines
• Multiple measurements: Maximum NT thickness from at least 3 good measurements should be used
• Amnion distinction: NT must be distinguished from the overlying amnion (two separate lines should be visible)

NT Percentile Table by CRL

CRL (mm)	GA (weeks+days)	Median NT (mm)	95th %ile NT (mm)
45	11+0	1.32	2.18
50	11+3	1.42	2.35
55	11+6	1.52	2.51
60	12+1	1.62	2.67
65	12+3	1.72	2.84
70	12+6	1.81	2.99
75	13+1	1.91	3.15
80	13+3	2.01	3.32
84	13+6	2.09	3.45

Worked Example

A 35-year-old woman undergoes first-trimester screening at CRL 60 mm with a measured NT of 2.8 mm:

The measured NT of 2.8 mm exceeds the 95th percentile for this CRL. Combined with the maternal age of 35 (background risk of approximately 1:250 for trisomy 21), this finding warrants further evaluation. Options include combined screening with serum markers (PAPP-A and free beta-hCG), cell-free DNA testing (NIPT), or diagnostic testing (chorionic villus sampling). The patient should be counseled about the difference between screening and diagnostic tests, and that most fetuses with mildly increased NT are chromosomally and structurally normal.

Frequently Asked Questions

What is a normal NT measurement?

Normal NT increases with gestational age (CRL). At 12 weeks (CRL ~60 mm), the median NT is approximately 1.6 mm. Measurements below the 95th percentile (approximately 2.7 mm at 12 weeks) are generally considered normal. An absolute cutoff of 3.0 mm is also commonly used. However, NT should always be interpreted in the context of CRL, maternal age, and other screening markers.

What if the NT is increased?

An increased NT does not mean the baby has a chromosomal abnormality. It means the risk is higher than average and further testing is recommended. Options include combined screening (NT + blood markers), cell-free DNA screening (NIPT), or diagnostic testing (CVS or amniocentesis). Approximately 90% of fetuses with NT between 3.0–3.5 mm are chromosomally normal.

When should NT be measured?

NT should be measured between 11 weeks 0 days and 13 weeks 6 days of gestation (CRL 45–84 mm). Before 11 weeks, the fetus is too small for reliable measurement. After 14 weeks, the lymphatic system begins to drain the fluid, causing NT to naturally decrease regardless of chromosomal status, which reduces the sensitivity of the test.

Does maternal age affect NT interpretation?

NT thickness itself is not affected by maternal age. However, maternal age is a critical factor in calculating the combined risk for chromosomal abnormalities. The background risk of trisomy 21 increases with maternal age (from approximately 1:1,500 at age 20 to 1:100 at age 40). NT screening modifies this age-related risk based on the MoM value. A woman with a high MoM will have her risk adjusted upward, while a low MoM will adjust the risk downward.

Is NT screening accurate?

NT measurement alone detects approximately 64–70% of trisomy 21 cases at a 5% false-positive rate. Combined first-trimester screening (NT + PAPP-A + free beta-hCG + maternal age) detects 85–90%. For the highest detection rate, cell-free DNA testing (NIPT) achieves >99% sensitivity, though it is a screening test (not diagnostic) and requires confirmation with CVS or amniocentesis.