Normal individuals have two copies, but those with Down's syndrome have three. Among the plasma samples from the women with fetuses affected by Down's syndrome, Dr. Lo and colleagues identified fetal cells with three chromosome 21 signals, the genetic marker for the disease. One candidate technique is the isolation of fetal cells from the cellular fraction of maternal blood, an approach that generally needs expensive and advanced equipment because of the rarity of such cells. Fetal DNA has been identified in the plasma of pregnant women, at concentrations much higher than those present in the cellular fraction, thus opening up new possibilities for non-invasive prenatal diagnosis. Until recently, it was assumed that fetal DNA detected in maternal plasma was cell free and a definitive diagnosis of fetal chromosomal aneuploidies was not thought possible.
Studies now indicate that some fetal DNA originates from intact fetal cells.
Researchers said their technique is simple for the prenatal detection of fetal trisomy 21 by FISH analysis of fetal cells harvested from maternal plasma, and it does not require expensive fetal cell enrichment equipment. Our data suggest that future large-scale trials should be initiated to assess the diagnostic accuracy of this method for fetal aneuploidy detection.