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Likelihood Ratio of Down Syndrome vs Normal Karyotype with use of Nuchal Thickness above Expected Value


Change in Nuchal Thickness (mm)

Likelihood Ratio



0 to <0.5


0.5 to <1.0


1.0 to <1.5


1.5 to <2.0





Expected Nuchal Thickness = 2.8444 - BPD x 0.0513 + BPD(2) x 0.0015

Likelihood Ratio = % Down syndrome / % Normals with given increased change in Nuchal Thickness Value

Risk = Maternal Age Risk x Likelihood Ratio eg 1/270 x 1.88 = 1/144

*Risk may be calculated from multiple sonographic markers by multiplying the likelihood ratios by the maternal age risk eg Maternal Age Risk x Humeral MoM, Nuchal MoM, Hypoplastic 5th Digit MoM, etc

Adapted from:Bahado-Singh R, Deren O, Oz U, Tan A, Hunter D, Copel J, Mahoney MJ. An alternative for women initially declining genetic amniocentesis: Individual Down syndrome odds on the basis of maternal age and multiple ultrasonographic markers. Am J Obstet Gynecol 179:514-9, 1998.

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Ultrasound Educational Press     Go Top
Peter W. Callen, M.D.
Professor of Radiology, Obstetrics, Gynecology and Reproductive Science
University of California Medical Center, San Francisco, California

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