Current prenatal diagnosis of fetal genetic status or aneuploidy depends on the use of invasive diagnostic tests which carry a small but significant risk of miscarriage. For many years, research has focused on the identification of fetal cells in the maternal circulation as a source of fetal material for safer noninvasive prenatal diagnosis (NIPD) but it is now recognised that this approach is unlikely to be clinically useful because of the paucity of these cells and the inability to analyse them reliably. In 1997, Lo et al. identified the presence of cell-free fetal DNA (cffDNA) in the maternal circulation. This emanates from the placenta, can be detected from 4 weeks of gestation and is rapidly cleared from the maternal circulation after delivery, making it a potential source of fetal material for prenatal diagnosis. However, the vast majority of cell-free DNA in the circulation is maternal in origin, with the fetal component contributing around 3% in early pregnancy, rising to 6% towards term. Current methodologies do not allow complete separation of fetal from maternal cell-free DNA and so current applications focus on the detection or exclusion of genes not present in the mother, such as Y chromosome sequences or rhesus D (RHD) in RhD-negative women. Following extraction of total cffDNA, a highly sensitive polymerase chain reaction (PCR) method called real-time PCR is used to amplify the gene in question; for example, RHD in an RhD-negative woman, or SRY or DYS14 if undertaking fetal sex determination. A positive signal would indicate the fetus was RhD-positive or male, respectively, but no signal would indicate that the fetus did not carry the target gene and was RhD-negative or female, respectively. Failure to obtain a signal could, of course, be a result of a failure to amplify the fetal DNA in the sample and work is continuing to identify universal fetal markers to use as internal standards.
This SAC opinion paper can be downloaded as a pdf below:
