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 cell was done via biopsy on day three of the embryo development; nowadays we know that a trophectoderm biopsy on day five 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 five. The results of the genetic analysis are usually available within 24 hours and therefore the healthy embryos can be transferred within the same cycle without the necessity to freeze them.
Can embryos after PGD be frozen?
Yes, through vitrification we can freeze all normal embryos, which have not been transferred.
What genetic information can we get from the embryos?
Depending on the indication, the following genetic studies can be done:
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.
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.