Preimplantation genetics and recovery of fetal cells from maternal blood.

Advances in molecular genetics have made possible advances in two new areas of prenatal diagnosis. Preimplantation genetic diagnosis is being explored through polar body biopsy, biopsy of the single cell from the eight-cell embryo, and trophectoderm biopsy of the blastocyst. There are attractive theoretical advantages to the last approach. However, assessment of pregnancies to be continued has been attempted only in the former two, and has succeeded only in the eight-cell embryo. A major problem with polar body biopsy is recombination for loci near the centromere, which may make analysis of the first polar body alone uninformative. DNA amplification of a single cell by polymerase chain reaction also has inherent pitfalls. Fetal cells in maternal circulation have ostensibly been shown to exist: lymphocytes, trophoblasts, and erythroblasts. The basis for this claim is polymerase chain reaction amplification showing fetal DNA sequences in maternal blood. Diagnosis of fetal chromosomal abnormalities requires techniques other than polymerase chain reaction, namely fluorescence in situ hybridization. Fetal trisomy has thus been detected by flow-sorting maternal blood for fetal erythroblasts followed by subjecting sorted cells to fluorescence in situ hybridization for chromosome-specific probes.