Cell-free foetal DNA in maternal plasma does not appear to be derived from the rich pool of cell-free foetal DNA in amniotic fluid.

Background: Large quantities of cell-free foetal DNA have been detected in amniotic fluid, and it has been proposed that this material may contribute to the pool of cell-free foetal DNA in maternal plasma.
Methods: Twelve maternal blood samples were obtained from pregnant women about to undergo an amniocentesis. Cell-free DNA was extracted from the maternal plasma samples and the matched amniotic fluid samples. The amount of cell-free foetal DNA was quantified by real-time PCR assays for the SRY and RHD genes.
Results: Amniotic fluid was found to contain vast quantities of cell-free DNA (median concentration = 3,978 copies/ml amniotic fluid). The concentration of cell-free foetal DNA in maternal plasma was much lower (median concentration = 96.6 copies/ml maternal plasma). No significant correlation could, however, be determined between these two pools of cell-free foetal DNA.
Conclusions: Our data confirm that amniotic fluid contains prodigious quantities of cell-free foetal DNA, but as no relationship exists between this material and that in the maternal circulation, it is unlikely that the amnion contributes to the presence of cell-free foetal DNA in maternal plasma.