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The trials performed worldwide toward noninvasive prenatal diagnosis (NIPD) of Down's syndrome (or trisomy 21) have shown the commercial and medical potential of NIPD compared to the currently used invasive prenatal diagnostic procedures. Extensive investigation of methylation differences between the mother and the fetus has led to the identification of differentially methylated regions (DMRs). In this study, we present a strategy using the methylated DNA immunoprecipitation (MeDiP) methodology in combination with real-time quantitative PCR (qPCR) to achieve fetal chromosome dosage assessment, which can be performed noninvasively through the analysis of fetal-specific DMRs. We achieved noninvasive prenatal detection of trisomy 21 by determining the methylation ratio of normal and trisomy 21 cases for each tested fetal-specific DMR present in maternal peripheral blood, followed by further statistical analysis. The application of this fetal-specific methylation ratio approach provided correct diagnosis of 14 trisomy 21 and 26 normal cases. [ABSTRACT FROM AUTHOR]