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Preimplantation genetic diagnosis was developed to reduce the risk of passing on a genetic disorder in high-risk couples. The aim was to enable them to start a pregnancy knowing that it was unaffected by a disorder. This provides an alternative to standard prenatal diagnosis which is carried out in an established pregnancy; a positive result in such cases meant that the couple would have to make an agonising decision regarding pregnancy termination.
Genetic testing of your embryos may be recommended if:
In the PGD procedure, going through routine IVF, several embryos are tested before being implanted in the womb, when they are three days old. The process includes ovary stimulation through medication, egg retrieval and fertilisation in the laboratory. Over the following three days, when the embryos reach 6 to 8 cells, it is safe to remove one or two cells for diagnostic testing, which takes 24 to 48 hours. Usually, embryos which pass the test are transferred to the mother five days after egg collection.
There are two main techniques used for diagnosis. In the case of single gene disorders like myotonic dystrophy or cystic fibrosis, DNA is extracted from the individual cells and analysed after amplification (increase in the number of copies) by the Polymerase Chain Reaction (PCR). For chromosomal disorders such as translocations or for sex-linked disorders such as hemophilia, fragile X- syndrome and most neuromuscular dystrophies, Fluorescent In Situ Hybridisation ( FISH) is used.
Once the PGD procedure is completed and embryos free of genetic problems have been identified, implantation will be attempted through embryo transfer, intracytoplasmic sperm injection (ICSI) or zygote intrafallopian transfer (ZIFT). While PGD helps reduce the chance of conceiving a child with a genetic factor, it can not completely eliminate this risk. In some cases, further testing is needed during pregnancy in order to determine the existence of a genetic factor.