Preimplantation genetic diagnosis (PGD) is a recently developed technique to determine genetic defects in embryos created by in vitro fertilization prior to implantation in a uterus for gestation. The diagnosis occurs at the 6 to 10 cell stage of embryonic development, when one or two cells can be removed without harm to the embryo or affecting its potential to implant. The cellular DNA is then tested for chromosomal abnormalities or genetic mutations, using polymerase chain reaction (PCR) for monogenic diseases and fluorescent in-situ hybridization (FISH) for chromosomal aberrations (Lissens and Sermon 1 99 7). Embryos carrying serious genetic diseases are discarded (or frozen indefinitely); embryos free of disease can be implanted for gestation. The procedure is still experimental, but there is little doubt that rapid developments in molecular biology will permit detailed genetic analysis of single embryonic cells in the foreseeable future, enabling PGD to be offered for wide range of conditions (Botkin 1998). PGD can be used as an alternative to traditional prenatal diagnosis (such as chorionic villus sampling 4 CVS 4 and amniocentesis) and selective abortion for couples at risk of transmitting inherited disorders. Because the affected embryos are discarded prior to implantation, PGD avoids the need for abortion, and many people find it preferable to traditional prenatal diagnosis for that very reason. For example, Lord Winston was cited in an article in The Times as saying,“Ethically it [PGD] is better than termination of pregnancy”(Murray 1998). However, this is an oversimplification.