Efforts to improve the means to diagnose and treat human genetic diseases have a long history in biomedical research and medicine. Now, preimplantation genetic diagnosis provides a new way to prevent the transmission of certain types of human genetic diseases to the next generation. It is an alternative to elective termination of pregnancies.PGD is used to test for genetic diseases that are due to defective single genes or abnormal chromosomes within days of fertilization and prior to the establishment of pregnancy. (...) The procedure essentially begins with the biopsy of one or more cells of a cleavage stage or blastocyst stage preimplantation human embryo that has been produced by in vitro fertilization. In certain cases, PGD can be done on polar bodies—discarded by-products of egg formation containing excess chromosomes—of unfertilized eggs. Then, the cell, or a polar body, is placed into a tube for single gene analysis ), or for chromosome analysis by spreading the nucleus of the cell on a microscope slide ). (shrink)