Preimplantation Genetic Diagnosis

What is the preimplantation genetic diagnosis (PGD)?

The preimplantation genetic diagnosis (PGD) is a diagnostic technique to detect genetic abnormalities in the embryo before transferring it into the woman’s uterus. To do so it is necessary to obtain a cell of the embryo to analyse its genetic material.

 

How are the embryo cells obtained?

Until recently, the removal of the embryo cells was done via biopsy on day three of the embryo development; nowadays we know that a trophectoderm biopsy on day 5-6 is more efficient and secure for the embryo.

For the biopsy of the trophectoderm a laser is used to cut into the pellucid zone of the embryo through which the cell is extracted for analysis.

When are the analysed embryos transferred?

The biopsy of the trophectoderm is made on day 5-6 of development. The results of the genetic analysis will be available within a few days and therefore the embryos need to be frozen after the biopsy has been performed. Once we get the genetic result, we can organise the cycle for the transfer of the normal embryos. 

Can embryos after PGD be frozen?

Yes of course. Via vitrification we can freeze all embryos without suffering any damages.

What genetic information can we get from the embryos?

Depending on the indication, the following genetic studies can be done:

Aneuploidy Study

Consists of analysing the number of chromosomes of the embryo applying Arrays CGH.

The transfer of embryos with genetic abnormalities might lower the pregnancy rates and increases the percentage of miscarriages. Detecting these embryos improves the results of an assisted reproduction program. Preimplantation genetic diagnosis (PGD) is recommended in those cases in which a chromosomal risk for the embryo exists, such us:

  • Couples with recurrent miscarriages 
  • Women of advanced reproductive age
  • Implantation failures
  • Genetic male factor

Structural chromosomal abnormalities

Occasionally we have healthy patients with alterations in their Karyotype who only notice problems when trying for pregnancy. Those patients who are carrier of a balanced or Robertsonian translocation as well as inversions might generate abnormal embryos. In these cases, a preimplantation genetic diagnosis (PGD) is indicated to identify the altered embryos and avoid their transfer.

Single-gene disorder

A different kind of diseases are those, which are produced by the mutation of a single gene. These alterations are not identifiable with the Karyotype and need analysis of the structure of a certain DNA string in order to be detected.

The transmission of certain genetic diseases through the parents can be avoided applying the preimplantation genetic diagnosis (PGD) on the obtained embryos within an assisted reproduction cycle. Some of these diseases are cystic fibrosis, beta thalassemia or any other muscle dystrophy.