When there is a history of transmissible genetic or chromosomal family illnesses, it can be very helpful to find out if an illness is present in the precursor cells (eggs), or in the embryo itself, before the woman becomes pregnant (before this embryo is transferred into the uterus).
In the cases where this is possible a technique called Pre-implantation Genetic Diagnosis (PGD) is used. These procedures are compulsorily carried out within the IVF programme in order to attain the genetic material that needs to be studied, the techniques of the Assisted Reproduction laboratory are required.
The diagnosis can be made in two different ways:
- Pre-implantation Genetic Diagnosis with pre-embryos – the embryos are studied before being transferred to the uterus.
- Pre-Implantation Genetic Diagnosis with eggs – the eggs are studied before being fertilised, that is to say, before the embryo is formed.
The couples who may need PGD are those where the woman is a carrier of chromosomal alterations (reciprocal and Robersonian translocations) and the man has a normal karyotype. Women in these cases often have repeated miscarriages.
What is the pre-implantation genetic diagnosis with embryos?
In certain cases where couples run the risk of the foetus having congenital alteration due to the woman’s history of family illness, Pre-implantation Genetic Diagnosis (when recommended) helps diagnose this alteration at a very early stage. With this technique the genetic or chromosomal alterations in the embryo can be detected before its transfer to the uterus. The procedure takes place in the laboratory after IVF when the embryos are at the 4-8 cell stage.
This technique allows the biologist to select and transfer to the uterus only those embryos diagnosed as normal.
After having been appropriately informed and having accepted to be included in an IVF programme, the woman undergoes hormonal induction to stimulate the follicles. All the retrieved mature eggs are inseminated to attain embryos. Twenty four hours later a biopsy is carried out on the embryos meeting the necessary requirements, and one of the cells is selected with a micromanipulator. The embryos are identified and are kept in an incubator while the study continues.
The obtained cells are prepared under specific conditions and their genetic material is studied. This allows the biologist to find out whether or not the embryo (from which the cell is taken) is affected by the illness. Once the state of each embryo is known, those free of the illness are selected to be transferred to the uterus. At the same time the affected ones are disposed of.
If there are no other sterility problems, and the age of the women is suitable, the chances of success should be comparable to conventional In Vitro Fertilisation.
What is the pre-implantation genetic diagnosis with eggs
In the case of Pre-implantation genetic diagnosis with embryos, the couple has to be aware of the fact that in order to have an illness-free pregnancy only healthy embryos are selected. This necessarily implies that the affected ones will be disposed of.
Preconception diagnosis with female gametes (eggs) is used to detect genetic or chromosomal pathologies even before the embryo is formed. In this case it is not necessary to select the embryos because an egg cell called second polar body is used. Only genetically healthy eggs are fertilised. This technique cannot be carried out on a spermatozoon and its application is limited to the inherited pathology of maternal origin.
The couples who may need PGD are those where the woman is a carrier of a chromosomal reorganisation (mainly reciprocal and Robersonian translocation) and the man has a correct karyotype, meaning his spermatozoa are chromosomally normal. In 91.4% of cases where this technique is used, a polar body with a good chromosomal morphology can be obtained.
The patient is informed about the procedure in detail and accepts being included in an in vitro fertilisation programme. In this case, she undergoes hormonal induction treatment to stimulate her follicles. A genetic study is carried out on all the obtained mature eggs. In order to do this, the polar body is biopsied by means of a micromanipulator (a tiny incision is made in the zona pellucida surrounding it) and is then identified and kept in the incubator.
To ensure the result, before a Preconceptional diagnosis is made it is important to try the reagents (called probes) in the lymphocyte preparation of the woman (blood cells).
Once the normal eggs are identified they are inseminated via intracytoplasmic sperm injection (ICSI). Therefore, all the resulting embryos are suitable to be transferred and none of them have to be destroyed. The unused ones can be frozen and used during possible future attempts.
This type of diagnosis can be chosen by the couples who prefer gamete selection to embryo selection for their own ethical and moral reasons. It allows many women who run the risk of producing eggs with chromosomal alterations to have a higher possibility of conceiving a child who will either be a balanced carrier or free of the pathology.