This article describes the individual sonographic markers used in the Genetic Sonogram Scoring Index. The importance of the clustering of markers forms the basis of the scoring index, such that individual markers are assigned point values based on their sensitivity and specificity in the detection of Down syndrome. The points acquired by each fetus are tabulated into a final "score." The performance of the scoring index in the detection of aneuploidy is presented. The clinical application of the scoring index for the detection of chromosomally abnormal fetuses in patients both at high- and low-risk for aneuploidy is discussed.