Preimplantation genetic diagnosis.

Backg round: Preimplantation genetic diagnosis (PGD) is used to analyze embryos before their transfer into uterus. It is suitable for a group of patients who are at a substantial risk of conceiving a pregnancy affected by a known genetic defect. PGD requires medically assisted reproduction, embryo biopsy of one or two cells and genetic analysis using either fluorescent in situ hybridization (FISH) or polymerase chain reaction. New technologies for PGD are now emerging. Array-based technologies allow simultaneous testing of aneuploidy and specific genetic diseases in each embryo. The main indications for PGD have been single gene disorders and in-herited chromosomal abnormalities. Preimplantation genetic screening (PGS) was introduced for aneuploidy screening, with the aim of replacing euploid embryos and increasing pregnancy rates in certain groups of patients undergoing in vitro fertilization procedures owing to infertility. Lately, several randomized control trials have failed to show that PGS on blastomeres using FISH method improved the delivery rate compared to the control group. The main reason is probably the natural occurrence of chromosomal mosaicism in the cleavage-stage embryo. Conclusions: Over the last two decades, PGD has been shown to be a reliable and safe genetic test for couples who are at risk of a specific inher - ited disorder. For PGS, the results from several ongoing randomized controlled trials performed at different cell biopsy stage, using array-CGH and SNP array will provide the data needed to evaluate the clinical efficacy.