Technique

1- In vitro fertilisation

To conduct this procedure, it is necessary to obtain embryos from the couple using IVF techniques even when there are no infertility issues.

2- Embryo biopsy

This is conducted three days after insemination, when the embryo has about 6-10 cells. It consists of extracting one or two cells from the embryo, but without affecting its normal development. Using a laser fitted to a microscope, the outer layer of the embryo (zona pellucid) is dissected and the biopsy is then performed. Once the biopsy has been performed, the embryo is maintained in culture until the time of transfer (day +5).

 

3 - Diagnostic genetic analysis

The biopsy obtained is then processed for analysis by means of Fluorescent In Situ Hybridization (FISH) or Polymerase Chain Reaction (PCR) depending on the disease being analysed.

FISH

Cytogenetic analysis of interphase nuclei allows us to detect both numeric and structural chromosome mutations. The FISH technique consists in applying DNA probes which are specific for the chromosomes being analysed and this enables the chromosomes analysed to be counted, detecting possible aneuploidy (missing or extra chromosomes).

In the case of chromosome reorganisation, the chromosomes analysed are only those involved in the reorganisation. For aneuploidy screening in IVF patients, the 13, 15, 16, 18, 21, 22, X and Y chromosomes are studied, which permits a large number of abnormalities to be ruled out.

FISH of interphase nuclei for chromosome 13, 16, 18, 21, 22 and 15 and sex chromosomes FISH of interphase nuclei for chromosome 13, 16, 18, 21 and 22

FISH of interphase nuclei for chromosome 13, 16, 18, 21 and 22 (right) and 15 and sex chromosomes (left)

PCR

The PCR technique is used for the diagnosis of monogenic diseases. With this process, the presence of the altered gene is detected using specific DNA sequence amplification.

The diagnostic efficiency with the PGD technique is about 95%.

4- Embryo transfer

The choice of which embryos to transfer is based on the genetic test results and the embryonic viability characteristics. If there are extra viable pre-embryos which are not transferred in this cycle these are cryopreserved for subsequent cycles.

After a PGD cycle, Prenatal Diagnostic testing is advisable during the first weeks of pregnancy.

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