Nuchal translucency screening is one of the most common first-trimester ultrasound measurements, and it's easy to leave the appointment with a number and no context for what it means. This calculator gives you that context — how your NT compares to the published normal range, and how background risk changes with maternal age — while being upfront about the one thing it deliberately does not do: compute your personal combined-screening risk.
How NT screening works
Nuchal translucency is measured by ultrasound between roughly 11 and 14 weeks of pregnancy (a crown-rump length, or CRL, of 45 to 84 mm), when the fluid space at the back of the fetal neck is at its most measurable. The normal range for NT increases slightly as the fetus grows, so a raw millimeter measurement means little without knowing the CRL it was taken at. Published reference data (Nicolaides et al., BMJ 1992) show the median NT rising from about 1.2 mm at a CRL of 45 mm to about 1.9 mm at a CRL of 84 mm, with the 95th percentile rising from about 2.1 mm to 2.7 mm over the same range. An NT at or above roughly 3.5 mm is generally treated as markedly increased regardless of exact CRL, because that absolute cutoff approximates the 99th percentile across the whole window.
In clinical practice, NT is rarely used alone. It is one of four inputs — alongside maternal age, PAPP-A, and free beta-hCG — to first-trimester combined screening, which produces a single numeric risk estimate for Down syndrome and other conditions.
Why this isn't the same as your clinic's combined test
The certified combined screening result you get from your clinic — a number like "1 in 1,200" — is produced by FMF-licensed software (Astraia, ViewPoint, or PRC) that applies cohort-calibrated Gaussian likelihood-ratio models to your specific NT, age, and serum marker values. Those model parameters are proprietary, validated on large clinical cohorts, and not published in a form that a public calculator can responsibly reimplement. Building a version anyway — guessing at the likelihood ratios and multiplying them together — would produce a number that looks like a real personalized risk but isn't validated as one, which is a serious problem for a test this consequential.
Instead, this calculator shows you two real, independently-cited facts: how your NT compares to the published normal range for your CRL, and the standard maternal-age background risk table used throughout obstetrics. These are genuinely useful for understanding your result, but they are not interchangeable with — and must never be multiplied together to approximate — your clinic's actual combined risk.
What happens if your NT is above range
Most pregnancies with an NT above the 95th percentile, even up to and including the 3.5 mm cutoff, turn out to be chromosomally normal. An elevated NT is a flag to discuss with your provider — not a diagnosis of anything. The typical next steps your OB or a genetic counselor may discuss include non-invasive prenatal testing (NIPT/cfDNA screening), which is substantially more sensitive and specific than NT alone, and diagnostic testing (chorionic villus sampling or amniocentesis) if a definitive answer is needed. A markedly increased NT is also associated with certain structural conditions independent of chromosomes, which is why a detailed anatomy ultrasound later in pregnancy is often recommended regardless of the screening result. Whatever the result, screening is optional and personal — there is no wrong choice, and a genetic counselor can walk through the options without pressure in either direction.