Age Adjusted Ultrasound Risk Assessment


Home > Calculators > Age Adjusted Ultrasound Risk Assessment .

Genetic sonography has been used to  adjust the age related a priori risk for Down syndrome , and increase the detection rate for Down syndrome [1, 2]. Some authors suggest that the genetic sonogram should not be used as a sequential test following serum biochemistry and question the association of most isolated soft markers  with Down syndrome in women at low risk  [3, 4]. Women with a high a priori risk for Down syndrome should consider amniocentesis for karyotyping regardless of a normal ultrasound [1, 3 ].

The calculator below may be used to estimate the risk for Down syndrome after a "genetic sonogram".
To use the calculator :
1. Enter the mid trimester risk for Down syndrome in the aprior risk directly, or  select the patient's age at the time of delivery and press use maternal age to use the values from The California Prenatal Screening Program Provider Handbook.
2. Select the markers (if any) found during the sonogram. Press "Calculate Likelihood Ratio" or enter the value for likelihood ratio directly.
3. Press "Calculate Posterior Probability" to obtain the patient specific risk for Down syndrome.

Calculation of Age Adjusted Ultrasound Risk Assessment

Mid trimester apriori risk of Down Syndrome is

 1 in  

Use midtrimester risk for Down syndrome by maternal age of  
Ultrasound Marker [6] * Likelihood Ratios  [7]
Nuchal fold
Thickened soft tissue at the fetal occiput
> = 6 mm between 15 to 20 weeks'
Hyperechoic bowel
Bowel echogenicity comparable to bone

Short humerus
Measured to Expected Humeral Length is < 0.9

Expected Humeral Length = -7.9404 + 0.8492 * BPD
Measured Humeral Length

Short femur
Measured to Expected Femur Length is <= 0.91

Expected Femur Length = -9.3105 + 0.9028 * BPD
Measured femur Length          
Echogenic intracardiac focus
Discrete echogenic spot as bright as bone
Anterior posterior dimension of the renal pelvis >= 4 mm between 16 to 20 weeks'
Total post-ultrasound likelihood ratio

Patient-specific risk for Down syndrome posterior probability                                           

1 in     *

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

Patient-specific risk for Down syndrome = O(MA)/LR + 1 − 1/LR
O(MA) is the odds of Down’s syndrome based on maternal
age and LR is the likelihood ratio [4].

The ultrasound findings must be independent from one another to justify multiplying them together. For example, humeral and femoral length are known to be correlated and the likelihood ratios for each of these should not be multiplied.

All calculations must be confirmed before use. The suggested results are not a substitute for clinical judgment. Neither 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.


1. Nyberg DA, et. al., Age-adjusted ultrasound risk assessment for fetal Down's syndrome during the second trimester: description of the method and analysis of 142 cases. Ultrasound Obstet Gynecol. 1998 Jul;12(1):8-14.PMID: 9697277

2. Aagaard-Tillery KM, et al. First and Second Trimester Evaluation of Risk Research Consortium.
Role of second-trimester genetic sonography after Down syndrome screening.Obstet Gynecol. 2009 Dec;114(6):1189-96.

3. Weisz B, Pandya PP, David AL, Huttly W, Jones P, Rodeck CH. Ultrasound findings after screening for Down syndrome using the integrated test. Obstet Gynecol. 2007 May;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 Jun;27(6):535-44. PMID: 17367102  

5. The California Prenatal Screening Program Provider Handbook 2009 California Department of Public Health. Genetic Disease Screening Program Provider Handbook 2009 (PDF, new window)

6. Bromley B, Benacerraf BR.The Genetic Sonogram Scoring Index. Semin Perinatol. 2003 Apr;27(2):124-9. PMID: 12769198

7. Smith-Bindman R, Hosmer W, Feldstein VA, Deeks JJ, Goldberg JD.Second-trimester ultrasound to detect fetuses with Down syndrome: a meta-analysis. JAMA. 2001 Feb 28;285(8):1044-55.PMID: 11209176


Home | About | Disclaimer | Privacy | Contact

Copyright © 2011 by Focus Information Technology. All rights reserved.