Preimplantation Genetic Screening – PGS

Genetic diagnosis in embryos before implantation in utero is a technology that helps couples who are in the process of in vitro fertilization (IVF) to improve the chances of the process and end it in a successful and normal pregnancy. According to many doctors, one of the main reasons for the failure of implantation in the uterus as part of the IVF procedure, is the implantation of a defective embryo, often one that carries an abnormal number of chromosomes.

Today, advanced laboratory methods make it possible to test the number of chromosomes presented in single cells. Performing the test helps the laboratory staff of the IVF unit select the most suitable embryos for implantation, to increase the chances of conceiving and the chances for healthy pregnancy ending in a live birth.

Using this technique may save the couple from multiple IVF rounds, that involve taking medications and ovule extraction that is usually performed under general anesthesia. Also, the procedure may reduce the risk of miscarriage – a not-so-simple experience that can be very disappointing especially for those who make great efforts to conceive.

Who is the procedure suitable for?

PGS may be suitable for women who have had miscarriages or couples who have experienced repeated failures in fertility treatments.

What are chromosomal disorders?

Each embryo begins its existence on the day of fertilization as a single cell, formed from the connection between sperm and ovule. A normal fetus receives 23 chromosomes from its father and 23 chromosomes from its mother (46 in total). Abnormal number of chromosomes – excess or deficiency, known as aneuploidy. This is a relatively common condition and many embryos, whether formed naturally or through in vitro fertilization, contain an abnormal number of chromosomes.

In most cases, an embryo containing an abnormal number of chromosomes will not normally implant in the uterus, what will end in a miscarriage during the first weeks of pregnancy. It is known that most natural abortions are of aneuploed embryos, that is, those that contain an abnormal number of chromosomes. There are also cases when an embryo with an abnormal number of chromosomes will take roots in the womb and be born, however the newborn will suffer from congenital malformations and developmental disorders.

PGS – the technology behind the procedure

A survey to detect chromosomal disorders in pre-implantation embryos (PGS), allows embryos to detect embryos with unbalanced changes in the number or structure of their chromosomes. This is done by identifying an excess or large deficiency of chromosome segments, in a single cell or group of cells taken from the embryo, in its early stages of development.

Cell sampling

For the PGS procedure, a sample must be taken from embryos that have successfully developed by the fifth day after fertilization and have become blastocysts. The group of cells being tested is separated from the whole embryo by a device called a micromanipulator. The cell or group of cells representing one particular embryo is transferred to a marked test tube. The test tubes are marked with the same marking as the embryo that allows accurate association of each sample to the embryo from which it was taken.

At this point, the test tubes containing the embryonic cells are transferred to a laboratory for genetic diagnosis of the embryos, for the purpose of molecular diagnosis. The embryos from which the cells were taken are kept in appropriate conditions in the laboratory of the IVF unit, or frozen until the genetic diagnosis process is completed.

The genetic material, the DNA extracted from the individual cell or group of cells sampled from each embryo, is replicated multiple times in a device called the Ploymerase Chain Reaction. The replicated sections are used for testing on the genetic chip.

Test on a genetic chip

The test is based on a special glass chip, on which are stamped small sections of the genetic material that represent the human genetic information. The genetic material of the subject, in this case from the embryo sample, is bound under controlled conditions to corresponding segments on the chip. Comparison between the sample and the chip is performed using unique software, which makes it possible to determine whether the tested sample has a lack or excess of an entire chromosome or of a large section of one of the chromosomes.

Test efficiency

It should be taken into account that since the amount of genetic material used for the test is minimal, and originates in one cell or in several cells, the effectiveness of the test is limited to detecting large changes only. A missing or excess of an entire chromosome or a very large part of it can be detected, but small changes can not be detected in this way. The method is based on a quantitative comparison between a normal human genetic material and the substance being tested, in this case of one cell or several cells is embryonic in its early stages of development. Therefore, balanced changes that do not cause a lack or excess of genetic material, such as balanced translocations, will not be detected in this test.