**2. Fetal cells and cell-free DNA**

The idea that genetic information of the fetus can be discovered by investigating maternal blood during pregnancy stems from the historic concept of Georg Schmorl, who described cross-placental trafficking of fetal cells into the maternal circulation. Fetal trophoblast cells were first demonstrated in lung tissue in mothers who died from eclampsia [3]. The isolation of fetal cells has remained a challenge due to their very low quantity [4,5], the limited knowl‐ edge on the characteristics and suboptimal markers for identification [6]. The focus has moved to the analysis of fetal cell-free DNA fragments which were first described in 1997 [1]. Cellfree DNA in maternal blood is comprised of extracellular DNA fragments that can be found in the maternal plasma and serum. The majority of cell-free DNA in maternal circulation is of maternal origin and around 10% is of fetal origin. Cell-free fetal DNA is released into the maternal circulation from cells of the placenta. It can be detected very early in pregnancy and is cleared a few hours after birth [7].

Initially, it was only feasible to analyze sequences of paternal origin and de novo mutations that were different from the maternal genome due to the high percentage of maternal cell-free DNA. Therefore, early studies focused on fetal Rhesus-status and on the detection of autoso‐ mal-dominant disorders of paternal inheritance [8]. Real-time quantitative PCR technology proved to be suitable for the detection of fetal loci that are different from the maternal genome such as the Y chromosome. Fetal gender determination was applied in families with a high risk for X-chromosome-linked disorders in which only male fetuses are affected from the disease and for the detection of fetal Rhesus D in pregnancies at risk for hemolytic disease of the newborn [9–11]. Just recently, non-invasive prenatal testing for routine fetal Rhesus D genotyping in Rhesus-negative women has been proven to be highly accurate over a 2-year period after its implementation in Denmark and proved to have the ability to direct the use of Anti-D Rhesus prophylaxis in prenatal care [12].

With the technique of next-generation sequencing, it is now possible to also reliably quantify specific DNA sequences and therefore assess sequences that are not only present in the fetus but also present in the maternal genome. This is accomplished by comparing the measured quantity with a reference genome, hence offering the possibility for the widespread analysis for the detection of most common fetal aneuploidies [13].
