When was the Pre-implantation Genetic Diagnosis used?
The first pregnancies obtained after a Pre-implantation Genetic Diagnosis (PGD) were in 1990 by a British group. Since then this is a diagnostic option offered in most countries where local legislation permits it. In Spain it is allowed in current legislation, and the first gestation was completed with the birth of two healthy girls in 1993.
What is a Pre-implantation Genetic Diagnosis?
This method allows for the earliest detection of genetic anomalies in the embryos. It consists of finding possible abnormalities in the embryos, even before pregnancy, in previous stages of lab implantation. This diagnostic method includes an in vitro fertilisation treatment which takes various embryos for analysis.
Who receives PGD?
In general, PGD is indicated in couples with a high risk of genetic anomalies in their offspring. Candidate couples usually have a unique reproductive history and the established indications include:
-Couples with previously affected children
- Women with history of abortions.
- Couples with an affected direct relative.
Currently these indications can be extended to cases such as:
- Women in an advanced reproductive stage (greater risk of transmitting abnormality to offspring).
Patients with repeated failure of in vitro fertilisation.
How is it performed?
Patients are submitted to a cycle of In Vitro Fertilisation (IVF). On the third day of the cultivation one or two cells are extracted (known as blastomeres) from each embryo in a process known as an "embryo biopsy." Proper extraction of the cell does not harm the embryo or its future development. The genetic information held in the extracted cell informs us of the genetic characteristics of the offspring relative to the disease tested. The cell is analysed with the aim of searching for the abnormality tested. Biopsied embryos are held in cultivation until the results are received. Currently this process can be completed and transferred on the same day or the following day. If the embryo carries the abnormality in mind, it is considered inept for transferring to the maternal uterus. Only those embryos without anomalies can be transferred to the mother.
What diseases can be diagnosed with this technique?
In theory, the problem with the PGD is that it can only diagnose the specific disease being searched for. Therefore diagnosis can not be discussed in general, but only for those diseases with a recognized diagnostic procedure. With this in mind, techniques can be used for:
-Detecting diseases linked to sexual chromosomes. Those diseases with a probability of transmission depending on the sex of the child (ex. Haemophilia, retinitis pigmentosa)
- Detecting diseases relative to abnormalities in the correct number of chromosomes. The most common cases affect the chromosomes 13, 16, 18, 21, 22, X, Y.
- Detect diseases when parents are believed to be carriers of some chromosome reorganisation: Translocations, deletions, inversions
- Detecting specific mutations affecting a single gene, like: Detecting cystic fibrosis for carrying F508 and 1609 from CA, etc.
Can PGD be used in selecting our child's sex?
Yes, but only when within the law: when having detected diseases whose heredity depends on the sex of the future child.
Does PGD substitute prenatal gene diagnostic techniques?
No, the two techniques are not exclusive. Prenatal gene diagnostic techniques (amniocentesis and/or corium biopsy) are indicated in all cases where a PGD is performed. This allows us to confirm the diagnosis obtained on the embryo through PGD.
The big advantage of PGD over other prenatal diagnosis techniques is its early selection of embryos (before implantation) while prenatal diagnostics are carried out with the fetus already in pregnancy. In the latter case, if a genetic abnormality is detected in the fetus, the patient is given the option to abort. Many parents do not object to this abortion, while others discard this option because of the ethical objections.
What probabilities are their of becoming pregnant?
In Europe, the registry on Pre-implantation Genetic Diagnostics on hereditary diseases goes back to 1999 in the European Society for Human Reproduction and Embryology (ESHRE). The last edition of the data from 2001 includes data from 25 centres in which 1561 PGD cycles were performed, registering 309 clinical pregnancies and 279 live births. This is an overall average of 20% pregnancies per PGD cycle.
