Enter a crown-rump length to see the expected NT, then enter your measured NT and gestational age to get a percentile and a simple normal range check.
What this tool calculates
- Panel 1 estimates expected nuchal translucency in millimetres from a given crown rump length in millimetres.
- Panel 2 compares a measured NT at a stated gestational age in days to reference centiles, then reports an approximate percentile and a simple range band.
Formulas and assumptions
- Expected NT from CRL: NT = 0.437 + 0.01969 × CRL(mm). Linear fit to median NT across CRL 45 to 84 mm within the usual 11 to 13+6 week window. Calibrated to FMF median tracks.
- Reference spread: SD(CRL) = 0.32 + 0.0028 × CRL, capped to 0.30 to 0.55 mm to avoid extreme tails.
- Percentile: z score equals measured NT minus mean, divided by SD at the matched CRL. Percentile equals Normal CDF of z.
- CRL to gestational age: first trimester dating based on Robinson and Fleming, restricted to the NT window.
Range bands shown
- Percentile lens: below the 95th percentile reported as within normal range. At or above the 95th percentile reported as above the 95th percentile.
- Absolute lens: bands used for counselling cut points, for example less than 3.5 mm, 3.5 to 4.4 mm, 4.5 to 5.4 mm, 5.5 to 6.4 mm, and 6.5 mm or more.
Scope and limits
- Use when CRL is 45 to 84 mm, which aligns with about 11+0 to 13+6 weeks for standard NT assessment.
- The percentile method assumes an approximately normal spread around the median at each CRL. Real world distributions can be skewed. Educational estimate only.
- Final interpretation uses maternal age, nasal bone, serum markers, and the full scan, with follow up testing per local protocol.
Sources
- Fetal Medicine Foundation, NT assessment and centiles. fetalmedicine.org/research/assess/nt and FMF course notes at courses.fetalmedicine.com.
- Robinson and Fleming CRL dating standard for first trimester gestational age from CRL. PDF chart mirror: bcwomens.ca CRL chart.
- UK screening materials on enlarged NT and common follow up thresholds around 3.5 mm. Example leaflet: portsmouth.gov.uk leaflet.
- Clinical digests on percentile or MoM approaches in first trimester screening. Example summary: The ObG Project.
Last updated: 26 Sep 2025
How to use this calculator
- Panel 1: enter crown-rump length in mm. The tool shows expected NT and derives gestational age in days. You can edit the age.
- Panel 2: enter measured NT in mm and gestational age in days to get a percentile and a normal range check.
- Outputs include expected NT, NT percentile, a within normal range flag, and a plot against 5th, 25th, 50th, 75th, and 95th curves.
Normal is defined as below the 95th percentile for the given age. Percentiles are population guides, not diagnoses.
Last updated: September 2025
Frequently asked questions
What is a normal NT measurement?
Normal is below the 95th percentile for the gestational age. The cut off in mm changes with age.
When should NT be measured?
NT is measured in the first trimester when CRL is in the standard NT range. The calculator estimates age from CRL so your result aligns with the reference curves.
What if my NT is at or above the 95th percentile?
This is an elevated risk context. Follow up may include a detailed scan and optional tests such as NIPT, CVS, or amniocentesis after clinical review.