**3.2. Targeted sequencing**

Targeted sequencing sequences only the regions / chromosome of interest and thus can be more time- and cost-efficient compared to whole genome sequencing. The principle is to selectively amplify the regions from chromosome 21, 13, and 18 followed by NGS. This method is also referred to as digital analysis of selected regions (DANSR). The amount of sequencing for a reliable detection is significantly lower around 40,000 and 1 million mapped sequences / sample. Unique to this type, the analysis uses a fetal fraction optimized risk score (FORTE) and takes into account the a priori risk (maternal age and gestational age) and uses an odds ratio approach to calculate the risk for aneuploidy.
