Pre-implantation genetic diagnosis (PGD)
PGD (Pre-implantation Genetic Diagnostic) — the testing of the embryo for chromosomal abnormalities and genetic diseases before transfer into the uterine cavity. Used with IVF and ICSI, when fertilization occurs outside the woman`s body.
The purpose of the procedure:
- Preventing the birth of a child with genetic abnormalities, that is especially important for high risk families.
- Improving the efficiency of assisted reproductive technologies.
- Reducing the risk of miscarriage or missed abortion.
What diseases can be detected using PGD
Using genetic diagnostic of the embryo, the following chromosomal diseases can be prevented in an unborn child:
- Patau syndrome (severe congenital malformations);
- Down syndrome (specific appearance, congenital malformations, mental retardation);
- Turner syndrome (short stature, developmental abnormalities, abnormal formation of the sex glands, the disease is associated with female sex);
- Edwards syndrome (gross development abnormalities);
- Kleinfelter syndrome (the disease is associated with male sex, an increase in the mammary glands, impaired sexual function) and others.
Moreover, it is possible to identify monogenic diseases (in the case when there is a high risk of inheriting a particular disease), for example:
- cystic fibrosis — dysfunction of the external secretion glands, respiratory and digestive tract;
- sickle cell anemia — a disturbance of hemoglobin structure, anemia with appropriate symptoms;
- hemophilia A — a coagulation factor deficiency associated with prolonged life-threatening bleedings;
- Duchenne muscle dystrophy — progressive muscle weakness;
- and others (the list of monogenic diseases studied includes more than 100 items; consult your doctor about the possibility of preventing a specific disease).
As you can see, all diseases are severe, and most of them are not amenable to correction: gross developmental anomalies lead to disastrous results in a short time; other pathologies reduce the quality of life of the whole family. Therefore, considering a high risk of such diseases in the family, the use of PGD in the IVF cycle is an important step in planning the birth of a healthy child.
Indications and implementation plan
Pre-implantation genetic diagnostic is performed if the couple has at least one of the following factors:
- The risk of genetic pathology — the spouses or their close relatives have hereditary diseases.
- The age of the mother at the time of IVF or ICSI is over 35 years, and age of the father is over 39 years.
- Significant deviations in sperm analysis.
- The couple had miscarriages and failed IVF attempts in the past. The reasons for this outcome were not identified.
Diagnostic is carried out at the specific time point after fertilization:
3rd day after fertilization
5-6th day after fertilization
At this stage, there are 6–8 blastomeres on which genetic information is “printed”. One of them is used for biopsy examination.
At the blastocyst stage is performed a biopsy of the outer shell, trophectoderm. The result is more accurate using this method.
Despite the need for biopsy, the embryo does not cause any harm, and the procedure itself does not affect the development of pregnancy. All cells at these stages are polypotent; it means that they are able for further development to a full-fledged organism.
Separately, it must be told about the examination of polar bodies. These are cells that are formed during the division of the female germ cell; they contain the same set of chromosomes as the ovocyte, and subsequently dissolve. The advantage of the method is that the embryo tissues are not used, because the biopsy is carried out before fertilization.
The geneticist determines after a number of preliminary procedures at what stage and for which specific diseases to conduct a genetic analysis of the embryo.
Do I need to prepare for PGD?
A specialist examines the already existing embryo; therefore, the participation of spouses is not required. Analysis of the polar bodies also includes taking material in advance. In these cases, the preparation is to follow the general rules that exist for IVF and ICSI.
The preparation is different if a couple has a high risk of chromosomal abnormalities. Both a man and a woman must donate blood to determine the karyotype. This will provide an opportunity to determine the set of tests for the embryo genetic diagnostic.
The basis of genetic diagnostics is PCR (polymerase chain reaction), that is analysis of a cell DNA. It consists in recreating a copy of the DNA region using enzymes and its analysis.
There is also a second method — fluorescent hybridization, when molecules capable for luminescence are fixed on DNA using a probe. Watching the material under a fluorescent microscope it is possible to count the exact number of chromosomes and identify any structural pathology.
The final stage of pre-implantation genetic diagnostic is the assessment of the embryos structure and selection of the embryo with the best morphological characteristics.
Diagnostic lasts from 1 to 30 days depending on the IVF protocol and the PGD method.
All these procedures can be performed using the cryoprotocol: a diagnostic first, then a freeze. The transfer is carried out in the next cycle. This approach allows transfer when the mother's body is no longer exposed to hormones for stimulation, which is beneficial for the expectant mother and for the child.
There is no interference with the functioning of the body of the father and mother during genetic diagnostic. The maximum that is required from the spouses — to donate blood from a vein. Therefore, the method is absolutely safe.
The issue of PGD safety for the embryo has also been deeply studied by the scientists. The experts concluded that the method does not affect the risk of developing congenital malformations or other abnormalities that may occur after birth based on the conducted studies. The procedure is carried out at the stage of development when the embryo cells have not acquired specific functions, so they can easily replace each other in the process of subsequent division.
Benefits of PGD
- there are no pregnancy termination; if there is a risk of having a child with a genetic pathology — this embryo is simply not used for transfer;
- a wide range of detectable diseases;
- high method sensitivity;
- the possibility of determining the sex of the child, and thus excluding the sex-linked diseases.