Down Syndrome: Age-Adjusted Ultrasound Risk Assessment (Nyberg AAURA)

Updated 5/25/2026

This calculator is updated to use the original Nyberg age-adjusted ultrasound risk assessment (AAURA) model for second-trimester Down syndrome risk estimation. Isolated soft markers may have limited impact in low-risk populations, and patients with high a priori risk should consider diagnostic testing regardless of a normal ultrasound. The calculator should not be used to "revise" a negative or positive cfDNA result by simply multiplying these historical ultrasound likelihood ratios.

How to use:
1) Use only when there has been no prior cfDNA/NIPT result. Enter the mid-trimester a priori risk (1 in N), or select maternal age and click "Use Maternal Age."
2) Select every ultrasound finding that is present.
3) Click "Calculate Likelihood Ratio" and then "Calculate Posterior Probability" to obtain the patient-specific risk (1 in N).
Enter the "N" from "1 in N" directly

Populates "1 in N" using the selected age's mid-trimester risk value.
Ultrasound markers (select any present). Likelihood ratios shown at right.
Long-bone entry instruction: The original Nyberg AAURA method multiplies likelihood ratios assuming marker independence. Because humeral and femoral shortening are correlated long-bone findings rather than statistically independent markers, multiplying both LRs may overestimate risk. If both are shortened, consider using only the stronger long-bone LR for clinical counseling, or clearly label the multiplied value as the historical Nyberg model estimate.
Nyberg structural category included cardiac defect, cystic hygroma/hydrops, cerebral ventricular dilatation, and duodenal atresia.
LR 25
Nuchal fold > 5 mm in the anteroposterior plane on the standard transcerebellar/off-axial view.
LR 18.6
Subjectively increased bowel echogenicity; moderate to markedly echogenic bowel (grades 2–3), approaching or equal to surrounding bone echogenicity.
LR 5.5
Measured/expected humeral length <= 0.89. Expected = -7.9404 + 0.8492 × BPD.
LR 2.5
Measured/expected femur length <= 0.91. Expected = -9.3105 + 0.9028 × BPD.
LR 2.2
Discrete bright focus within the heart, usually left ventricle.
LR 2.0
Renal pelvis of either kidney > 3 mm in the anteroposterior plane.
LR 1.6
Use only when no structural abnormality and none of the listed non-structural markers are present.
LR 0.4
LR is the product of selected Nyberg marker LRs; this follows the original AAURA method but assumes independence.
Nyberg multiplication model
Output is "1 in N" patient-specific risk after ultrasound findings.
Notes & assumptions

This page uses the original Nyberg AAURA assigned likelihood ratios: structural defect 25, nuchal thickening 18.6, echogenic bowel 5.5, short humerus 2.5, short femur 2.2, echogenic intracardiac focus 2.0, renal pyelectasis 1.6, and normal ultrasound 0.4.

Patient-specific risk (legacy formula from the original page):
Risk (1 in N) = O(MA)/LR + 1 − 1/LR
Where O(MA) is the odds based on maternal age (or other a priori risk) and LR is the likelihood ratio.

Nyberg AAURA multiplied likelihood ratios when more than one marker was present. However, long-bone markers can be correlated; the multiplied LR should therefore be interpreted as the historical Nyberg model rather than a fully validated independent-marker estimate. This calculation should not be applied after a negative or positive cfDNA/NIPT result without separate counseling because cfDNA substantially changes the pretest/post-test framework.

Sensitivity, false-positive rate, and maternal age ≥40 limitation

In the original Nyberg AAURA cohort, using a 1:200 threshold identified 74% of fetuses with trisomy 21 and produced an overall false-positive rate of 14.7% among controls. Performance varied by maternal age: the false-positive rate was lower in women under 35 years and substantially higher in older age groups because the calculation begins with maternal-age pretest risk.

A normal ultrasound was assigned LR 0.4 in the Nyberg model. This can reduce estimated risk, but it does not exclude trisomy 21. The original paper specifically notes that women age 40 years or older remained above a common invasive-testing threshold despite a normal ultrasound because the baseline age-related risk was high.

This is a historical pre-cfDNA risk model. It is best presented as an educational age-adjusted ultrasound risk estimate when no cfDNA/NIPT result is available, not as a replacement for modern screening or diagnostic testing.

Disclaimer: All calculations must be confirmed before use. The suggested results are not a substitute for clinical judgment. Neither Perinatology.com nor any other party involved in the preparation or publication of this site shall be liable for any special, consequential, or exemplary damages resulting in whole or part from any user's use of or reliance upon this material.
References
  1. Nyberg DA, et al. Age-adjusted ultrasound risk assessment for fetal Down's syndrome during the second trimester. Ultrasound Obstet Gynecol. 1998;12(1):8-14. PMID: 9697277
  2. Weisz B, et al. Ultrasound findings after screening for Down syndrome using the integrated test. Obstet Gynecol. 2007;109(5):1046–1052. PMID: 17470581
  3. Smith-Bindman R, et al. Second trimester prenatal ultrasound for the detection of pregnancies at increased risk of Down syndrome. Prenat Diagn. 2007;27(6):535-44. PMID: 17367102
  4. Bromley B, Benacerraf BR. The Genetic Sonogram Scoring Index. Semin Perinatol. 2003;27(2):124-9. PMID: 12769198