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Preimplantation Genetic Diagnosis

Preimplantation Genetic Diagnosis is a technique adjunct to In Vitro Fertilization with ICSI that is used to select embryos genetically and/or chromosomically healthy for their transfer into the uterus. This technique helps to increase the chances of achieving pregnancy through Ivf treatment and prevents the occurrence of hereditary diseases. Therefore, the embryos selected for transfer or freezing are selected according to three parameters: morphological, kynetic and genetic, thus increasing pregnancy rates with reduced risk of abortion.

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Indications

  • Advanced maternal age (> 40 years)
  • Repeated spontaneous abortions
  • Patients with pregnancies carrying previous genetic diseases
  • Repeated implantation failure
  • Severe male factor (altered FISH)
  • Monogenic disease carriers (disease caused by mutation in a specific gen)
  • Carriers of chromosomal reorganizations in the karyotype (translocations, inversions)

Steps

1. Embryo biopsy

To obtain genetic information from each embryo a cell should be removed from each one of them (blastomere). Embryo biopsy is mostly carried out on d+3 of development, but it can also be performed on d+5. Experience shows that the removal of only one cell does not jeopardize embryo development as it is performed with complex micromanipulation equipment that permits safe cell removal.

Firstly, a small opening is made in the outer layer (zona pellucida) and then one blastomere is aspirated with a micropipette. The embryo is immediately returned to the incubator to continue its development. Meanwhile, the removed cell is appropriately treated to analyze its genetic and chromosomic content.

2. Chromosomic or genetic analysis

Chromosomic study through CGH arrays:

The use of molecular techniques like DNA arrays (DNA microchip) for Preimplantation Genetic Diagnosis has widened the study of the 23 pairs of chromosomes. Some years ago, embryo genetic diagnosis only permitted the study of 9 of the 23 pairs of chromosomes. But thanks to the introduction of DNA arrays through Compared Genetic Hybridisation (CGH) the information obtained from each embryo is much more complete and safe. Through CGH, genetic material from the embryo under study can be compared with “normal” genetic material to detect “gains” and “losses” that could prevent the embryo from developing and forming a healthy baby.

CGH arrays are specially used for aneuploidy screening (abnormalities in the number of chromosomes in an embryo), but they can also be combined with the specific study of a certain monogenic disease or chromosomic reorganization.