Nuchal Translucency Calculator
Enter your baby's crown-rump length (CRL) and nuchal translucency (NT) measurement from your 11-to-14-week ultrasound scan. The calculator computes the expected median NT for that CRL, your NT percentile and multiple of the median (MoM), the gestational age in weeks and days from CRL, and an adjusted risk estimate for trisomy 21 combining your maternal age with the NT measurement. All results update as you type.
What is nuchal translucency?
Nuchal translucency (NT) refers to the fluid-filled space at the back of a developing baby's neck, visible on ultrasound between 11 and 14 weeks of pregnancy. All fetuses accumulate some fluid in this area, but in chromosomally abnormal pregnancies - particularly those with trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), trisomy 13 (Patau syndrome), and Turner syndrome - the fluid tends to be thicker than usual. An NT scan is offered as part of routine first-trimester screening in many countries. It is a screening test, not a diagnostic test: a normal NT is reassuring but does not guarantee a chromosomally typical pregnancy, and an elevated NT requires further evaluation rather than being conclusive on its own.
How this calculator works
Enter the crown-rump length (CRL) and NT measurement from the ultrasound report, together with your age at delivery. The calculator first derives the expected (median) NT for that exact CRL using the regression equation NT = 0.437 + 0.01969 x CRL (mm), calibrated to the Fetal Medicine Foundation reference population. It then expresses the measured NT as a multiple of the median (MoM) and converts the deviation into a percentile using the normal distribution. Gestational age in weeks and days is estimated from CRL via the Robinson and Fleming formula (GA days = 8.052 x sqrt(CRL x 1.037) + 23.73). Finally, the background maternal-age risk for trisomy 21 is adjusted by an NT likelihood ratio based on how many standard deviations the measured NT falls above or below the euploid median, producing a combined first-trimester risk estimate.
What the percentile means
The percentile tells you where the NT measurement falls in the distribution of chromosomally normal pregnancies at the same CRL. A measurement at the 75th percentile means 75% of normal pregnancies have a lower NT at that gestational age; a measurement at the 97th percentile means only 3% of normal pregnancies are higher. The conventional clinical threshold is the 95th percentile, which corresponds approximately to an NT of about 2.5-3.0 mm depending on the CRL. An NT below the 95th percentile is considered in the normal range. Measurements at or above the 95th percentile are typically reported as elevated and prompt further counselling and testing, while those at or above the 99th percentile (or a fixed 3.5 mm cutoff used by some protocols) carry a substantially higher chromosomal risk and are managed more urgently.
Combined first-trimester screening
NT measurement alone detects approximately 64-70% of trisomy 21 cases at a 5% false-positive rate. Adding maternal serum markers - pregnancy-associated plasma protein A (PAPP-A) and free beta-human chorionic gonadotropin (beta-hCG) - raises detection to roughly 85-90% at the same false-positive rate, which is why clinical guidelines from the Fetal Medicine Foundation, ACOG, and NICE recommend the combined test rather than NT alone. Nasal bone assessment, ductus venosus flow, and tricuspid regurgitation can be added to push detection further. This calculator models the NT component of that risk; a full combined screening result requires the serum markers to be entered into a certified clinical algorithm.
NT thickness and associated outcomes (Souka et al. 2004)
| NT range | Category | Chromosomal risk | Fetal loss risk | Major anomaly | Favourable outcome |
|---|---|---|---|---|---|
| < 3.5 mm | Low risk | Background | Background | Background | > 95% |
| 3.5-4.4 mm | Mildly elevated | ~21% | ~2.7% | ~10% | ~70% |
| 4.5-5.4 mm | Moderately elevated | ~33% | ~3.4% | ~18.5% | ~50% |
| 5.5-6.4 mm | Significantly elevated | ~50% | ~10% | ~24% | ~30% |
| >= 6.5 mm | Markedly elevated | ~64% | ~19% | ~46% | ~15% |
Approximate population-level outcome estimates for pregnancies with increased NT in the absence of aneuploidy. Individual risk depends on many factors. Source: Souka AP et al., Outcome of pregnancy in chromosomally normal fetuses with increased nuchal translucency in the first trimester, Ultrasound Obstet Gynecol 2004.
Frequently asked questions
What is a normal NT measurement?
Normal NT varies with gestational age. Rather than a single fixed number, clinicians compare the measurement against the expected median for the given crown-rump length. A result below the 95th percentile is generally considered normal. In absolute terms, most guidelines treat any NT under 3.0-3.5 mm (depending on the CRL) as low risk, and several protocols use 3.5 mm as the threshold above which chromosomal assessment is recommended regardless of percentile.
What does an elevated NT mean?
An NT above the 95th percentile, or above 3.5 mm in absolute terms, increases the statistical probability of chromosomal abnormalities - most commonly trisomy 21, trisomy 18, trisomy 13, Turner syndrome (45,X), and triploidy. It is also associated with structural heart defects and rare genetic syndromes even when chromosomes are normal. An elevated NT does not mean a diagnosis: many pregnancies with mildly elevated NT have completely normal outcomes. The result needs to be interpreted together with serum markers, maternal age, and, if indicated, invasive testing such as chorionic villus sampling (CVS) or amniocentesis.
What is NT MoM and why does it matter?
MoM stands for multiple of the median. An NT MoM of 1.0 means the measurement equals the median for that CRL. A MoM of 2.0 means the NT is twice the expected median, which indicates significant elevation. MoM is used because it adjusts for the natural increase in median NT with gestational age, making measurements from different weeks directly comparable. Most combined screening algorithms use NT MoM (or an equivalent delta-NT or z-score) rather than the raw mm value when computing chromosomal risk.
What is the CRL range for NT scanning?
The NT scan must be performed when the CRL is between 45 mm and 84 mm, corresponding to gestational ages of approximately 11 weeks 0 days to 13 weeks 6 days. Below 45 mm, the NT space is too small to measure reliably; above 84 mm, the fetus begins to clear fluid from the neck naturally, making a normal result harder to interpret. Outside this window, the NT measurement is not used for chromosomal screening.
How does maternal age affect the risk calculation?
The risk of trisomy 21 rises with maternal age, approximately doubling every five years after age 30. A 25-year-old has a background risk of about 1 in 1,300; by age 35 it is roughly 1 in 365; by age 40 it is close to 1 in 100. The NT measurement modifies this background risk: a low NT reduces it (making an already-low risk even lower), while an elevated NT multiplies it (making it higher). The combined first-trimester risk figure is produced by multiplying the background risk by the NT likelihood ratio, which is why the same NT value carries very different implications for a 22-year-old versus a 42-year-old.
Is a normal NT result enough to rule out Down syndrome?
No. NT screening alone has a detection rate of roughly 64-70% for trisomy 21 at a 5% false-positive rate. This means approximately 30% of affected pregnancies are missed when NT is used alone. Combined first-trimester screening (NT + PAPP-A + free beta-hCG) raises detection to about 85-90%. For the highest sensitivity, cell-free fetal DNA (NIPT/cfDNA) testing can be added or used independently. No screening test is 100% sensitive; diagnostic certainty requires CVS or amniocentesis with fetal karyotyping.
What happens after an abnormal NT result?
An elevated NT result is followed by counselling with a maternal-fetal medicine specialist or genetic counsellor. Options discussed typically include combined serum screening if not already done, NIPT (which analyses cell-free fetal DNA from a maternal blood sample with high accuracy), detailed anatomical ultrasound at 18-20 weeks, fetal echocardiography, and invasive diagnostic testing (CVS or amniocentesis) which provides a definitive chromosomal result. The pathway depends on the degree of NT elevation, the combined risk estimate, local guidelines, and the family's preferences.
Sources
- Snijders RJ, Noble P, Sebire N, Souka A, Nicolaides KH. UK multicentre project on assessment of risk of trisomy 21 by maternal age and fetal nuchal-translucency thickness at 10-14 weeks of gestation. Lancet 1998.
- Souka AP, Von Kaisenberg CS, Hyett JA, Sonek JD, Nicolaides KH. Increased nuchal translucency with normal karyotype. Am J Obstet Gynecol 2005.
- Fetal Medicine Foundation. Nuchal translucency scan and first-trimester combined screening.