Calculator

Genetic sonography has been used to adjust age-related a priori risk for Down syndrome and improve detection in some settings. 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.

How to use:
1) Enter mid-trimester a priori risk (1 in N), or select maternal age and click “Use Maternal Age”.
2) Select markers found. Click “Calculate Likelihood Ratio” (or enter LR directly).
3) Click “Calculate Posterior Probability” to obtain the patient-specific risk (1 in N).
Enter the “N” from “1 in N”.
Populates “1 in N” using the selected age’s mid-trimester risk value.
Ultrasound markers (select any present). Likelihood ratios shown at right.
Thickened soft tissue at the fetal occiput ≥ 6 mm between 15–20 weeks.
LR 17
Bowel echogenicity comparable to bone.
LR 6.1
Measured/Expected humeral length < 0.9. Expected = -7.9404 + 0.8492 × BPD.
LR 7.5
Measured/Expected femur length ≤ 0.91. Expected = -9.3105 + 0.9028 × BPD.
LR 2.7
Discrete echogenic spot as bright as bone.
LR 2.8
AP renal pelvis diameter ≥ 4 mm between 16–20 weeks.
LR 1.9
LR is the product of selected marker LRs (assumes independence; see Notes).
Assumes marker independence
Output is “1 in N” patient-specific risk after ultrasound findings.
Notes & assumptions

The likelihood ratio for a US marker is defined as sensitivity/false-positive rate.

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.

Assumption: ultrasound findings must be independent to justify multiplying LRs. Some findings are correlated (e.g., humeral and femoral length); multiplying their LRs may overestimate risk change.

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. Aagaard-Tillery KM, et al. Role of second-trimester genetic sonography after Down syndrome screening. Obstet Gynecol. 2009;114(6):1189–96. PMID: 19935018
  3. Weisz B, et al. Ultrasound findings after screening for Down syndrome using the integrated test. Obstet Gynecol. 2007;109(5):1046–52. PMID: 17470581
  4. 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
  5. California Prenatal Screening Program Provider Handbook (2009), California Department of Public Health, Genetic Disease Screening Program. Provider Handbook 2009 (PDF)
  6. Bromley B, Benacerraf BR. The Genetic Sonogram Scoring Index. Semin Perinatol. 2003;27(2):124–9. PMID: 12769198
  7. Smith-Bindman R, et al. Second-trimester ultrasound to detect fetuses with Down syndrome: a meta-analysis. JAMA. 2001;285(8):1044–55. PMID: 11209176