Down syndrome risk calculator

First-trimester trisomy 21 risk estimate based on maternal age, crown-rump length and nuchal translucency.

Appears only on the printed report. It is not sent or stored.

Ultrasound data (11 to 13 weeks 6 days)

Enter the crown-rump length and nuchal translucency from your first-trimester scan. If you have not had the scan yet, tick the option above to calculate from age only.

How the risk is calculated

The calculation starts from the maternal age-related trisomy 21 risk, adjusted for gestational age at the scan, and adds 0.75% when there is a previous child with Down syndrome. The nuchal translucency is then converted to a multiple of the median (MoM) for the crown-rump length and applied as a likelihood ratio: the larger the NT relative to the median, the higher the final risk.

The result guides the conversation with your obstetrician: high risk usually leads to further investigation with NIPT or an invasive test (chorionic villus sampling or amniocentesis). An increased NT also calls for assessment of the fetal heart and other anomalies.

Model and references

This calculator follows the first-trimester method used by the Fetalmed Down syndrome calculator: maternal age, crown-rump length, nuchal translucency and a previous affected child.

  1. Wisser J, Dirschedl P, Krone S. Estimation of gestational age by transvaginal sonographic measurement of greatest embryonic length. Ultrasound Obstet Gynecol. 1994;4(6):457-62.
  2. Cuckle HS, Wald NJ, Thompson SG. Estimating a woman's risk of having a pregnancy associated with Down's syndrome using her age and serum alpha-fetoprotein level. Br J Obstet Gynaecol. 1987;94(5):387-402.
  3. Hecht CA, Hook EB. The imprecision in rates of Down syndrome by 1-year maternal age intervals: a critical analysis of rates used in biochemical screening. Prenat Diagn. 1994;14(8):729-38.
  4. Snijders RJ, Sundberg K, Holzgreve W, Henry G, Nicolaides KH. Maternal age- and gestation-specific risk for trisomy 21. Ultrasound Obstet Gynecol. 1999;13(3):167-70.
  5. Nicolaides KH, Snijders RJ, Cuckle HS. Correct estimation of parameters for ultrasound nuchal translucency screening. Prenat Diagn. 1998;18(5):519-23.

Reference calculator: fetalmed.net/calculadora-risco-sindrome-down

Risk bands follow the reference calculator. Some services use a 1/300 cutoff for diagnostic testing.

Independent reproduction of the published models, for educational purposes.

Frequently asked questions

It means that among 100 pregnancies with the same characteristics, one is expected to have trisomy 21. It expresses probability, not a diagnosis.

No. Without the ultrasound, the calculator shows only the age-related risk, which is less accurate. Measuring NT between 11 weeks and 13 weeks 6 days substantially improves the estimate.

No. The calculation assesses only the trisomy 21 risk. Other chromosomal conditions and malformations are not evaluated by this tool.

Between 11 weeks and 13 weeks 6 days, when crown-rump length is between 45 and 84 mm. Outside this window the nuchal translucency measurement is no longer valid.

Important notice

This calculator is an educational tool and does not replace a medical consultation. Results are estimates based on published models and may not apply to your case. Never make decisions about your health or your pregnancy without advice from your doctor.

Reviewed by: Dr. Peterson Fernandes — CRM-MG 42452 · RQE 37814

Last reviewed: 11/3/2026

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