This diagnostic tool enables us to study the embryo for a particular condition before it is transferred to the uterus. This prevents certain inherited genetic disorders or abnormalities that may compromise embryo viability from being passed on.
When is it recommended?
This is used in some cases of:
- Repeated miscarriage
- Repeated failure of implantation
- Advanced maternal age
This involves studying a certain number of chromosomes (currently 9: X,Y,13,15,16,17,18,21, and 22), which are the locations of the most commonly found aneuploidies. This is done thanks to different techniques of genetic diagnosis (FISH, gene sequencing, microarrays, etc.).
Once it has been confirmed that the embryo is normal (i.e., two copies of each chromosome), it is selected for transfer.
The presence of balanced chromosomal translocations in one partner increases the risk of chromosomal alterations in offspring, causing miscarriage or severe birth defects or deformities.
Nowadays, PGD enables us to select embryos, discarding those that are chromosomally unbalanced. This requires that the couple undergo a previous peripheral blood test called an informativity study, which will give us an indication as to the reliability of the technique for the individual case being studied. Once the treatment has been given and the embryos have been studied, we will be able to transfer embryos that are normal for the rearrangement in question. As a result, no distinction will be able to be made between balanced translocation carriers and non-carriers.
There are certain severe X-linked recessive diseases that cause women to pass down the disease and males to be affected by the it.
In these cases, the approach taken involves performing PGD to distinguish male embryos (XY) from female embryos (XX) and transfer only those that are female (XX). This makes it possible to transfer embryos that will not develop the disease even though they will be carriers. Although there are many such diseases, the most well-known X-linked diseases are the following:
- Duchenne muscular dystrophy
- Lesch-Nyhan syndrome
- Hunter syndrome
Nowadays, a wide range of serious genetic disorders caused by mutations are candidates for PGD in the embryo prior to transfer, thus guaranteeing that the disease in question will not be passed on to subsequent generations. The basis of this test is the same as in other PGDs, and includes an informativity study, embryo screening, and transfer of healthy embryos. The diseases for which PGD is most commonly performed are as follows:
- Cystic fibrosis
- Huntington's disease
- Fragile X syndrome
- Steinert's disease
- Tuberose sclerosis
- Polycystic kidney disease
This is an open-ended list that will be updated as new information comes to light. Couples should consult the list to see if the disease affecting their family has been added and determine whether they may benefit from advances made in genetic diagnosis.
What does it involve?
To carry out a PGD, embryos must first be formed in the laboratory, meaning the mother-to-be must have an assisted reproductive technique (IVF or ICS) performed.
The study is done on one cell from this embryo. A hole is made in the outer layer of an embryo 3 days after fertilization (when it has 6-8 cells).
One cell (blastomere) is removed through this opening to determine whether it is normal or abnormal. The embryos found to be appropriate are selected for transfer.
How effective is it?
The PGD program's overall success rate per transfer is 20-30%, although these percentages vary based on what motivated the test.