Preimplantation Genetic Diagnosis (PGD) is the study of chromosomal and genetic abnormalities in embryos before they are transferred to the mother’s uterus by an In Vitro Fertilisation (IVF) technique. It seeks to ensure healthy offspring and stop transmission of a given disease.
Studying chromosomal anomalies in embryos is indicated in cases of structural chromosomal alterations, recurrent miscarriage, implantation failure (couples who have failed to achieve pregnancy after several In Vitro Fertilisation (IVF) cycles with good quality embryos and no evident causes) or severe male factor. These couples are more likely to produce chromosomally abnormal embryos, which become blocked, fail to implant or cause miscarriages. In these cases, by selecting chromosomally normal embryos by PGD we maximise the chances of achieving a full-term pregnancy and welcoming a baby at home.
Human beings, and therefore embryos too, have 46 chromosomes, 23 of them are inherited from the mother and the other 23 from the father. Embryos that have an excess or a deficiency in any of those chromosomes fail to implant or cause a miscarriage.
Until recently, the most widely used technique for PGD to diagnose chromosomal numerical abnormalities in embryos was fluorescence in situ hybridisation (FISH). By means of this technique, cells extracted from an embryo are tagged with fluorophore to make certain chromosomes visible. We can study 9 chromosomes with FISH (X, Y, 13, 15, 16, 17, 18, 21 and 22), which are the ones that appear more frequently associated with miscarriages and serious foetal malformations, thus increasing the rates of long-term pregnancy and healthy babies. Despite its advantages, this technique has a very important limitation: it cannot detect other chromosomes simultaneously, and the rest of the chromosomes remain unanalysed.
To solve this limitation, a new technology called array-CGH (Comparative Genomic Hybridisation in DNA chips) has been incorporated to PGD over the last year. This state-of-the-art-technique has one enormous advantage over FISH: all the chromosomes in an embryo can be viewed from one of its cells. In this way, we can find out if there is an abnormality in one of them, either a gain or a loss, and ensure the transfer of normal embryos for all the chromosomes.
At Instituto Bernabéu, a pioneering centre in the implementation of genetic and molecular techniques, this technology is available to those patients who need it, after an individual study of each case has been performed. Pregnancy rates are thus increased in a very important group of infertile patients as those who have suffered implantation failures and recurrent miscarriages, for whom there is a greater risk of producing embryos with an abnormal number of chromosomes. This technique could be the only solution they have to their fertility problem.
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