Because what matters is inside, the best embryos cannot always be detected upon simple visualization. In some cases it is necessary to analyse their chromosomal content using techniques such as the Preimplantation Genetic Diagnosis, which facilitate pregnancy upon detecting what embryos have the greatest chances of development.
What happens in Preimplantation Genetic Diagnosis?
Preimplantation Genetic Diagnosis (PGD) is technique that is performed on the embryo before it is implanted in the womb. PGD provides information on the condition of each of the embryos conceived and only allows the healthy ones to be transferred to the womb. The PGD technique is the result of the combination of in vitro fertilisation and genetic analysis.
How is PGD?
In Vitro Fertilisation to obtain embryos.
Embryo Biopsy. The embryo biopsy takes place 72-76 hours after recovery of the eggs, when the embryo is at the 6-8 cell stage. It consists of extracting one cell from the embryo without this compromising its normal development. Once the biopsy has been performed, the embryo is returned to the incubator in the Laboratory, where it will be kept in an in vitro culture with appropriate environmental conditions for continuing its development until the time of transfer to the patient’s womb.
Diagnostic Genetic Analysis. The cell obtained from the biopsy is processed for analysis and subject to genetic study by means of Array-CGH or FISH-24.
Embryo Transfer. The result of the genetic analysis is transmitted to the assisted reproduction team in the form of a detailed report and it is decided which embryos are going to be transferred depending on the chromosome pool and the morphological characteristics of embryonic viability. The transfer is made immediately after analysis, usually 5 days after recovery of the eggs.
Can the embryo be harmed by extracting a cell from it?
When the biopsy is performed, each embryo cell contains all the genetic information, so extraction of a cell does not affect the correct development of the rest of the embryo cells.
This technique requires high precision, and when carried out by experts it does not alter the development of the embryo. The risk of embryonic block is 0.67%.
In couples with infertility problems, who benefits from a PGD cycle?
It is advisable to request this technique in the following cases:
Women over 37.
Sterile patients with two or more IVF cycles without pregnancy.
Patients with repeated miscarriage (with 2 or more previous miscarriages).
Patients with a severe male factor.
Patients with altered meiosis or altered FISH in spermatozoa.
Patients with altered karyotypes (carriers of chromosomal reorganizations).
Why is this study important?
Most embryos with chromosomal abnormalities (embryos with extra or missing chromosomes):
DO NOT lead to pregnancy.
Are implanted but result in spontaneous miscarriage in the first trimester of pregnancy.
A small percentage of them develop and lead to a child affected by a disease.
Therefore, Preimplantation Genetic Diagnosis benefits a lot of the patients who consult infertility centres since:
The possibility of pregnancy is increased.
The risk of spontaneous miscarriage is reduced.
The risk of having to have a termination is reduced.
Which are the most common numerical chromosome abnormalities?
The most common chromosomal syndromes that can lead to a child affected by some form of disease are:
Down’s Syndrome (trisomy 21)
Patau Syndrome (trisomy 13)
Edwards Syndrome (trisomy 18)
Klinefelter Syndrome (47, XXY)
Turner’s Syndrome (45, X)
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