Screening for fetal chromosome aneuploidies in high risk pregnancies has for decades been offered in many countries. Until now the sampling of fetal material for this purpose has been done by invasive procedures such as chorionic villus sampling or amniocentesis, which carry a 0.5–2% risk of unintended termina- tion of the pregnancy .
Due to this risk it has long been of great interest that both circulating fetal cells and cell-free fetal DNA can be found in maternal blood [2,3]. It has been envisioned that these fetal compounds can be utilized for a non-invasive and therefore risk- free prenatal diagnostic procedure. The development of such procedures has, however, been hampered by the fact that both cell-free fetal DNA and circulating fetal cells are very rare in blood samples from pregnant women [4,5]. Furthermore, both cell-free fetal DNA and fetal cells are found among excessive amounts of both maternal cell-free DNA (5–10 fold excess) and maternal
blood cells (10E8–10E9 fold excess). The vast excess of maternal cells may be overcome by antibody-based fetal cell enrichment since different cell types express different cell type surface epitopes. This is in contrast to cell-free fetal DNA, which is biologically and chemically very similar to cell-free maternal DNA, making it difficult to obtain efficient enrichment of the fraction of cell-free fetal DNA.