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.
Press "Calculate Posterior Probability" to obtain the patient specific risk for
The likelihood ratio for a US marker is defined as
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 .
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 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.
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
Role of second-trimester genetic sonography after Down syndrome screening.Obstet
Gynecol. 2009 Dec;114(6):1189-96.
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
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.