PGD stands for preimplantation genetic diagnosis. The terms PGS (preimplantation genetic screening) and PGT (preimplantation genetic testing) are also used. There are 3 indicators that PGD can tell you about.
The first is an assessment of the number and quality of chromosomes in the embryo, in other words, an assessment of the chromosomal status. All hereditary information in every cell is contained in DNA, which is packed into 46 chromosomes. PGD can rule out or confirm the presence of chromosomal abnormalities such as Down syndrome, Patau syndrome, Edwards syndrome, and Klinefelter syndrome.
The second indicator is the presence or absence of monogenic diseases in the embryo. For example, cystic fibrosis, phenylketonuria, and others. In total, there are more than 3000 such diseases. The chromosome set in monogenic diseases corresponds to the norm, but the production of some proteins that are responsible for certain functions of the body is impaired. Testing for monogenic diseases must be done if both parents are carriers of the monogenic disease. In this case, the diagnosis is carried out for abnormalities of a particular gene.
The third indicator is translocation. Translocations are chromosomal abnormalities, which consist in the fact that during the process of cell division, part of one chromosome joins with part of another chromosome. For example, part of the fifth connects with part of the ninth, and the second part of the ninth with another part of the fifth. Such translocations can significantly affect human reproductive functions and the viability of the embryo.
All types of PGD are performed in the clinic according to the following scheme. First, superovulation, follicle puncture and fertilization of the resulting eggs are stimulated in the woman. All the resulting embryos are cultured to the blastocyst stage (this is the stage at which the embryo is implanted into the woman’s endometrium).
The blastocyst consists of an average of 100 cells. Some of the cells form the trophectoderm (future placenta), some – the inner cell mass (future fetus). Embryologists perform a biopsy of the trophectoderm, i.e. several (3-5) cells are taken from it, without damaging the future fetus, and sent for genetic analysis. The embryo is frozen. The procedure is carried out on high-tech equipment under a microscope using micromanipulators and a special laser device. In this case, both the embryo and part of the cells sent for genetic analysis are marked strictly in accordance with the belonging of one to the other.
After that, some of the cells of the trophectoderm are analyzed by geneticists for the presence of chromosomal abnormalities, monogenic diseases, or translocations. For each embryo, the geneticist gives a conclusion, on the basis of which the reproductive specialist, together with the patient, will be able to make a decision on the transfer of a specific embryo. On average, the PGD procedure takes 1 month.
What are the alternatives to PGD? There are technologies that can also tell about genetic abnormalities in the fetus.
It can be concluded that PGD has undeniable advantages over alternative diagnostic methods: