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