 The investigators have planned and developed the following approach: fetal cells are first enriched from blood of pregnant women, between 7 and 12 weeks gestation, employing the ISET isolation by size of epithelial tumor cells technique. Cells presumed to be of fetal origin are microdissected and subsequently genetically analyzed, using STR markers, to verify their fetal nature. The investigators then plan to test two strategies in order to assess the number of copies of chromosome 21. The first one involves the DNA of a single fetal cell to be analyzed with CGH comparative genomic hybridization array. In fact, our team has already developed an application of the metaphase CGH method to single cells isolated by ISET in which we were able to demonstrate the gain of chromosome 21 DNA in single fetal cells isolated from cord blood of a fetus with Down syndrome. The second strategy will be accomplished with the use of quantitative fluorescent PCR analysis of short tandem repeats STRs, applied to single cells. At least 5-8 highly polymorphic STR markers specific for chromosome 21 will be tested to minimize the effects of a phenomenon called allele drop out, in which one allele fails to amplify, and to maximize the number of triallelic signals for an accurate diagnosis of disomy or trisomy 21. 