**3.3. SNP-based sequencing**

This third approach was the most recent method introduced to the variety of clinically available NIPT options. This technique involves targeted amplification and sequencing of singlenucleotide polymorphisms (SNPs). SNPs are single base pairs that occur approximately once / 300 base pairs on the human genome and can be used to distinguish individuals. In addition to the above mentioned applications, maternal and fetal DNA also can be distinguished by SNP analysis. For this analysis, both maternal DNA from white cells from the buffy coat and maternal plasma which includes fetal and maternal DNA are used. In the SNP-technology originally introduced by Zimmermann et al. [16], 19,488 SNPs on the chromosomes 21, 13, 18, X, and Y are analyzed simultaneously. Taking into account the parental genotype, the fetal fraction, and the fetal chromosome copy number, billions of possible genotypes at a specific locus are considered by a complex algorithm and the observed allele distributions are com‐ pared to the expected allele distributions. By this method, the most likely fetal genotype can be calculated and a specific risk score for the analyzed aneuploidies is reported [16–19].
