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Pre-implantation genetic diagnostics (PGD)

Pre-implantation genetic diagnostics (PGD)

If the parents have genetic disorders or other health issues, to have a healthy child it is necessary that the embryo be tested by a geneticist prior to implantation into the uterus. The PGD (pre-implantation genetic diagnostics) arises out of the necessity to examine embryos produced by parents with genetic disorders.

IVF baby

Indications for pre-implantation genetic diagnostics

  • history of bearing children with genetic abnormalities
  • gene mutation presence in the couple
  • frequent spontaneous abortions/non-developing pregnancies
  • age of older than 38 
  • failure of several IVF attempts in the absence of visible negative factors

Reliability of embryonal genetic diagnostics

The difference between genetic testing of a human and of an embryo consists in the number of cells available for analysis. In case of genetic diagnostics of a human, there are thousands of cells available, which make the analysis very reliable. For embryonal diagnostics, only one cell, or at best five cells may be used. Even the smallest error in the technology, especially random DNA contamination*, will lead to erroneous results. Unfortunately, such errors do occur. To assure that the results are really obtained on the embryonal DNA rather than randomly introduced, often both parents’ DNA is analyzed with the use of individual markers of the parents’ DNA.

*Contamination is impurities, mixing and foreign inclusions.

How the PGD is performed

The material for PGD is taken by means of embryonal biopsy. The PGD (pre-implantation genetic diagnostics) always consists of embryonal biopsy and DNA analysis. If the diagnostics is performed by means of a polymerase chain reaction (which is always the case in genetic mutation diagnostics and in the analysis of all 24 chromosomes), a blastomere (the embryo’s cell) is removed for the analysis. The cell is taken either on the third day when the embryo consists of about eight cells, or on the fifth day when it consists of about 100 cells. If the biopsy is performed on the third day, only one cell is usually taken, if on the fifth day - up to five cells are taken. The possibility of taking more cells on the fifth day is an undeniable advantage of the later biopsy, and these days many clinics begin performing a biopsy at this later time. The problem, however, is that the PGD diagnostic part may take more than a day, and since the embryos are usually cultivated for not more than 5 days, they will have to be frozen. Even with the current reliable and safe embryonal freezing methods, such as vitrification, any procedure with an embryo involves the risk of its damage.

PGD safety for the embryo

The blastomere is taken when the cells of the future child are still universal and undifferentiated, and any of the cells can be replaced with others without causing damage to the child’s health. In most clinics, the embryonal biopsy is currently performed by a laser. Until recently, the biopsy itself was believed not to influence embryonal development, but as blastocyst* stage biopsy spreads, it is increasingly believed to be the safest option for the embryo. The blastocyst is an embryo on the 5th or 6th day of its development.

The diseases detected using PGD may be classified as follows:

  • hereditary genetic disorders (primarily gene disorder - related)
  • chromosomal disorders, i.e., those occurring spontaneously de novo, such as chromosomal loss or addition. For example, Down’s syndrome (three chromosomes number 21). Since this type of disorders represents the most frequent cause of spontaneous abortions or implantation failures, in theoretical terms, their diagnosis would increase the probability of normal pregnancy, although this is difficult to confirm in practice.

Although PGD was initially developed for diagnosis of hereditary genetic disorders, it was then adapted for embryonal selection aimed at improving the IVF treatment results through detection of chromosomally abnormal embryos. It should be noted that embryonal cell removal for analysis is a delicate process requiring experience and knowledge. A technical error during the biopsy may reduce the viability of an embryo, i.e., its ability to be implanted into the uterus. This should be borne in mind when defining indications for PGD. The world’s first PGD was performed on a rabbit’s embryo in 1967. Much time passed before it was performed on a human because of many technical and ethical issues. PGD initially arose out of the necessity to test embryos for gene mutations, when a family has a history of a certain genetic disorder. In 1988, the group led by Alan Handyside in the UK was the first to detect mucoviscidosis using PGD. Since chromosomal disorders are considered as the principal problem hindering pregnancy, PGD was quickly adapted for embryonal selection aimed at improving the IVF treatment results through detection of embryos with chromosomal abnormalities. Most of the chromosomal disorders in embryos are random, unpredictable, uncontrollable and occur in the final stages of the ova maturation. The probability of such disorders increases as a woman ages, and currently a woman’s age is the most frequent reason for PGD recommendation

Pre-implantation genetic screening (PGS)

To avoid confusion between family mutations and non-family (de novo) chromosomal disorders, the diagnosis of the latter is now called “pre-implantation genetic screening” (PGS). PGS was initially used for analyzing only two or three chromosomes, but in the last 20 years, the number of chromosomes tested has gradually increased, and some laboratories have begun offering their patients analysis all 24 chromosomes (22 + ХУ).At the same time, PGD reliability and usefulness for detecting embryos with family mutations is of no doubt. The reason for higher PGD reliability is that besides the embryo’s DNA, both parents’ DNA is also analyzed to confirm the diagnosis and exclude any error.