Genetics
Preimplantation genetic testing (PGT)
Stages of development of the human embryo
Preimplantation Genetic Testing (PGT) identifies and prevents chromosomal and genetic diseases that may be passed to the offspring prior to the stage of embryo transfer to the uterine cavity.
Naturally, human being’s genome has 46 chromosomes (23 pairs) received from both parents. Numerical disorders of chromosomes is a fairly frequent event in which an embryo can get an extra chromosome (trisomy), or a certain chromosome (monosomy) may be lost during the process of fertilization.
Most often, during preimplantation testing, 5 chromosomes (13, 18, 21, X and Y) are analyzed, their abnormalities lead to the development of most dangerous diseases in the future child. Abnormalities in other chromosomes may be the cause of non-viable embryos transfer, or may be the reason for the early pregnancy loss.
Most common reason for the development of chromosomal abnormalities is the «defect» in parental germ cells. At the same time, 85% of this abnormality occurs during egg maturation; 15% of cases are caused by spermatozoa chromosomal abnormalities. Most chromosomal abnormalities (95%) occur spontaneously.
Preimplantation genetic testing methods:
To this date the following methods of preimplantation genetic testing are distinguished:
The most modern and most accurate method of preimplantation genetic testing (PGT-A) an embryo aneuploidy is the next generation sequencing (Next Generation Sequencing) -NGS. The main task of PGT-A is to assess an embryo in its early stages of development (detection of numerical chromosome abnormalities – aneuploidy), selecting a healthy embryo for being transferred.
This significantly increases the chance of pregnancy and reduces the number of fails in IVF cycles. The time that takes to get pregnant and give birth to a genetically healthy child is reduced.
PGT-A is especially recommended for:
- Women aged 35 years and older;
- Men aged over 39 years;
- Recurrent IVF Failures (more than two);
- Miscarriage (recurrent pregnancy loss -more than two spontaneous abortions);
- Types of male infertility associated with chromosomal abnormalities;
- Carriage of chromosomal rearrangements, translocations, inversions and other disorders.
- Basing on indications, testing of embryos on 46 chromosomes can be combined with the diagnostics of monogenic diseases and HLA testing
Benefits of using PGT-A testing
- Increased chances of getting pregnant on transferring one genetically healthy embryo 78.6%; reducing number of «failed» transfers;
- Examination of all 23 pairs of chromosomes (excluding the risk of having a baby with chromosomal abnormalities);
- Testing accuracy 99.9%;
- Reducing the risk of spontaneous abortion due to a fetus’s chromosomal anomaly;
- Increased chances of giving birth to a healthy child in case of patients at a certain risk (late reproductive age, recurrent miscarriage, carriage of translocations, failed IVF attempts)
- The risk of chromosomal abnormalities can exist in case of all families and all embryos. The frequency of chromosome number abnormalities increases with a mother’s age.
- The chromosomal status of an embryo does not correlate with the morphology of an embryo.
What are the steps in PGT-A?
- Embryo biopsy is performed at the blastocyst stage on the 5-7 day of embryo development. This is an important stage for performance of laser-assisted trophectoderm biopsy to get sampled 5-7 cells which would serve to construct the placenta in the future. Internal cells from which an embryo’s organs and tissues would develop are not subject to be tested, therefore the method is safe.
- Vitrification (freezing) embryos;
- DNA extraction and whole genome sequencing;
- DNA sequencing result analysis;
- Medical and genetic counseling based on the results of the tests (doctor consults and gives an expert opinion on each embryo being analyzed);
- Transfer of healthy and diagnosed embryo transfer in a cryo-cycle.