terican

Last verified · v1.0

Calculator · health

Nuchal Translucency Trisomy 21 Risk Calculator

Estimate Trisomy 21 risk using nuchal translucency thickness, crown-rump length, and maternal age with the FMF-validated MoM formula.

FreeInstantNo signupOpen source

Inputs

Adjusted Trisomy 21 Risk (1 in N)

—

Explain my result

0/3 free

Get a plain-English breakdown of your result with practical next steps.

Adjusted Trisomy 21 Risk (1 in N)—

The formula

How the
result is
computed.

Understanding Nuchal Translucency Screening

Nuchal translucency (NT) screening is a first-trimester ultrasound procedure performed between 11 weeks 0 days and 13 weeks 6 days of gestation, corresponding to a fetal crown-rump length (CRL) of 45 to 84 mm. The measurement captures the maximum thickness of the fluid-filled subcutaneous space at the back of the fetal neck. Elevated NT is one of the most powerful early ultrasonographic markers for chromosomal aneuploidy, particularly Trisomy 21 (Down syndrome), and forms the backbone of first-trimester combined screening worldwide.

The Three-Step Formula

Step 1: Expected NT by CRL

The gestational-age-adjusted expected NT is calculated as: Expected NT (mm) = 1.2 + 0.018 x (CRL - 45). This linear regression equation reflects the physiological increase in NT thickness as the fetus grows. At a CRL of 45 mm (approximately 11+0 weeks), the expected NT is 1.20 mm. At 65 mm (about 12+1 weeks), it is 1.56 mm. At 84 mm (13+6 weeks), it reaches 1.92 mm.

Step 2: Multiples of Median (MoM)

MoM = Observed NT / Expected NT. Expressing the raw measurement as a MoM standardizes results across the 11-to-14-week scan window, allowing direct comparison between fetuses scanned at different gestational ages. A MoM of 1.0 represents the population median; values above 2.5 MoM fall in a statistically elevated risk range for chromosomal abnormality.

Step 3: Adjusted Trisomy 21 Risk

Adjusted Risk = Background Age-Related Risk / (MoM cubed). The background risk is the maternal-age-specific prevalence of Trisomy 21 at term. Cubing the MoM amplifies the discriminatory signal of the NT measurement: a MoM of 2.0 shifts the background risk by a factor of 8, while a MoM of 3.0 shifts it by a factor of 27.

Input Variables Explained

  • Crown-Rump Length (CRL), 45-84 mm: Measured in a neutral mid-sagittal plane from the top of the fetal head to the base of the spine. Measurements outside this range yield unreliable NT risk estimates and fall outside the validated gestational window.
  • Observed NT Thickness (mm): Recorded by a trained sonographer using the FMF-standardized protocol — neutral fetal neck posture, magnified image with the head and thorax filling the screen, and inner-to-inner caliper placement at the widest point of the translucency.
  • Maternal Age at Delivery: The mother's age on the estimated due date, not the scan date. Background Trisomy 21 risk rises from approximately 1 in 1,068 at age 25, to 1 in 249 at age 35, to 1 in 30 at age 45, reflecting the well-documented relationship between advanced maternal age and meiotic non-disjunction.

Worked Example

A 33-year-old woman attends her 12-week scan. The sonographer records a CRL of 72 mm and an observed NT of 3.4 mm. Step 1: Expected NT = 1.2 + 0.018 x (72 - 45) = 1.2 + 0.486 = 1.686 mm. Step 2: MoM = 3.4 / 1.686 = 2.02. Step 3: Background risk for age 33 is approximately 1 in 592. MoM cubed = 8.24. Adjusted risk = 1 in 72. This result exceeds the standard 1-in-100 high-risk threshold and warrants clinical discussion about confirmatory diagnostic testing.

Risk Category Thresholds

  • High Risk (1 in 100 or greater): Referral for chorionic villus sampling (CVS) or amniocentesis is typically recommended to obtain a definitive chromosomal diagnosis.
  • Intermediate Risk (1 in 101 to 1 in 1,000): Cell-free DNA (cfDNA) testing or enhanced surveillance may be offered as a non-invasive next step.
  • Low Risk (below 1 in 1,000): Routine antenatal care continues with standard follow-up scanning.

Methodology and Authoritative Sources

The expected NT regression formula and MoM-based risk adjustment align with methodology validated across large obstetric cohorts. A large cohort study published on PubMed Central established population-level normal reference ranges for NT thickness by CRL, confirming that MoM standardization significantly improves chromosomal risk stratification compared to unadjusted raw measurements. The three-step risk calculation mirrors the internationally recognized Fetal Medicine Foundation (FMF) first-trimester screening algorithm, the benchmark protocol used clinically in more than 50 countries. Center-specific median calibration — an essential quality-control process ensuring local ultrasound departments apply their own NT median rather than a generic population average — is detailed in peer-reviewed research on delta-NT and center-specific NT medians. Broader evidence on the precision and downstream health-economic impact of first-trimester screening is documented in research from Stanford University on targeting precision medicine through prenatal screening.

Disclaimer: This calculator provides educational risk estimates only. All results require interpretation by a qualified healthcare professional within a complete clinical context, including serum markers (PAPP-A and free beta-hCG), scan quality grading, and full patient history.

Reference

Frequently asked questions

What is a normal nuchal translucency measurement?
A normal NT measurement is generally below 3.0 mm between 11 and 14 weeks, though the expected thickness increases with gestational age. On the MoM scale, values between 0.5 and 2.0 are considered within the normal range. An NT at or above the 95th percentile — roughly 2.5 MoM or a raw value of 3.0 to 3.5 mm depending on gestational age — is associated with increased chromosomal and structural cardiac risk and typically prompts referral to a maternal-fetal medicine specialist for further evaluation.
How does maternal age affect Trisomy 21 risk in this calculator?
Maternal age determines the background risk of Trisomy 21, which rises steeply after age 35. At age 25 the term risk is approximately 1 in 1,068; at age 35 it is roughly 1 in 249; at age 40 it is about 1 in 59; and at age 45 it reaches approximately 1 in 30. The NT-based MoM value then adjusts this baseline figure upward or downward. A younger mother with a high MoM can reach a higher adjusted risk than an older mother whose fetus has a reassuringly low NT measurement.
What does a MoM value above 2.5 mean on a nuchal translucency calculator?
A MoM above 2.5 means the observed NT thickness is more than 2.5 times the expected value for that gestational age, which represents a significant elevation. This level is associated with substantially increased risk for Trisomy 21, Trisomy 18, Turner syndrome, and structural cardiac defects. For example, a fetus at 12+3 weeks with an NT of 4.0 mm against an expected value of 1.6 mm has a MoM of 2.5. Most clinical guidelines recommend prompt referral for diagnostic testing — CVS or amniocentesis — when MoM values reach this threshold.
At what gestational age should the nuchal translucency scan be performed?
NT screening must be performed between 11 weeks 0 days and 13 weeks 6 days of pregnancy, corresponding to a fetal CRL of 45 to 84 mm. Outside this window the measurement is unreliable: before 11 weeks the NT space is too small to measure accurately, and after 14 weeks the fluid is physiologically reabsorbed. Most guidelines recommend scheduling the scan between 12 and 13 weeks to maximize image quality, measurement accuracy, and the time available to arrange follow-up diagnostic testing if the result is high risk.
How accurate is the nuchal translucency calculator for detecting Down syndrome?
NT measurement alone, as modeled in this calculator, detects approximately 70 to 75 percent of Trisomy 21 cases at a 5 percent false-positive rate. When combined with serum markers PAPP-A and free beta-hCG in full first-trimester combined screening, detection rates rise to 85 to 90 percent at the same false-positive threshold. The Fetal Medicine Foundation has validated these performance benchmarks across tens of thousands of pregnancies, and they represent the current standard of care in first-trimester screening programs throughout Europe, North America, and Australia.
What should be done after a high-risk result from the nuchal translucency calculator?
A high-risk result of 1 in 100 or greater warrants urgent referral to a maternal-fetal medicine specialist or fetal medicine unit. Available options include chorionic villus sampling (CVS), performed from 11 weeks and returning a definitive chromosomal diagnosis within 2 to 3 days, or amniocentesis from 15 weeks with comparable diagnostic accuracy. Cell-free DNA testing offers a non-invasive alternative with sensitivity above 99 percent for Trisomy 21, though it remains a screening rather than a diagnostic test. A genetic counselor should guide the decision-making process based on individual clinical circumstances.