ANDREA L. BONNICKSEN
In vitro fertilization (IVF) clinics in North American and European countries have added to their service options the diagnosis of microscopic human eggs and embryos for genetic anomalies. In preimplantation genetic diagnosis (PGD), genes associated with serious disorders are detected before implantation and pregnancy. Preimplantation diagnosis is an alternative to prenatal (fetal) testing for couples who would not terminate a pregnancy but who are at high genetic risk for passing on such disorders as Tay Sachs disease or cystic fibrosis to their offspring. Embryo diagnosis is an early step in a line of practices that may one day include the genetic correction of embryos.
There are several types of PGD. In the embryo biopsy, a single cell from a four- to eight-cell externally fertilized egg is removed and examined. The embryo itself is transferred to the woman's uterus for a possible pregnancy only if the biopsied cell is free from the DNA sequence associated with the disease in question. In the trophectoderm biopsy, cells surrounding the multicell and older (but still microscopic) embryo are examined. This procedure leaves the cells of the inner embryonic mass intact, but successful implantation rates appear to be lower when embryos are transferred at this later stage. In polar body analysis, technicians examine the first polar body of the unfertilized egg. In situ hybridization is used to identify female and male embryos in order to transfer only female embryos to women at risk for passing a sex-linked genetic disease, such as