PGS: Preimplantation genetic screening

Preimplantation genetic screening, which is performed during IVF treatment, allows the choice of a perspective embryo with a normal number of chromosomes for transfer to the uterus. Such an embryo then has the greatest chance to be implanted and will not be miscarried and will lead to the birth of a healthy child.

Chromosomes

Chromosomes are fibrous formations that carry genetic information. A healthy person has 22 pairs of chromosomes (one pair from the mother and the other from the father) and two sex chromosomes that determine the sex of the individual.

Why are chromosomes so important?

Many IVF cycles may end up in disappointment because the implantation of the embryo fails. The wrong number of chromosomes of the embryo that was transferred into the mother’s uterus may be one of many causes of failure of an IVF cycle. A condition where the embryo has some extra chromosomes or some chromosomes are missing is called aneuploidies.

• Most pregnancies with aneuploid embryos end in spontaneous miscarriage in the first trimester.
• Some types of aneuploidy in embryos may lead to the birth of a handicapped child, such as Down syndrome (surplus chromosome 21), Edward’s syndrome (surplus chromosome 18) or Patau’s syndrome (surplus chromosome 13)
• With increasing age, the risk of aneuploidy also increases for women. Doctors are convinced that this is the main reason why there is a sharp decline in a woman’s fertility during the third and early fourth decade of life.

The screening of aneuploidies in embryos before transferring them to the mother’s uterus, which takes place during the IVF cycle, makes it possible to choose to transfer only embryos without any identifiable genetic abnormality. The purpose of this investigation is to increase the chance of giving birth to a healthy child and a successful IVF cycle.

Purpose of PGS

The purpose of screening chromosomes is to achieve the birth of a healthy child in the shortest possible timeframe using the safest medical procedures.

• Reduce the number of spontaneous miscarriages.
• Reduce the risk of pregnancy complications.
• Shorten treatment time and reduce the financial costs associated with the birth of a healthy baby by reducing the number of repeated cycles.
• Reduce the number of multiple pregnancies examined through the transfer of embryos that have a chance to lead to a healthy child.
• Provide important diagnostic information to the patient on the basis of which she may opt for another IVF cycle or ask for an egg donation.

Contribution of PGS to the outcome of IVF

In a normal IVF cycle without PGS testing an embryo selected on the basis of morphological criteria or according to the dynamics of development is transferred. Unfortunately, these criteria are not sufficient for us to reliably exclude embryos with genetic abnormalities.

Thanks to PGS we can check the genetic makeup of the embryo and then select the appropriate embryo to transfer.

Who is PGS for?

Preimplantation genetic screening is for:

• women of higher ages
• repeated spontaneous miscarriages
• repeated unsuccessful embryo transfers in previous IVF cycles
• miscarriage or delivery of foetus with developmental defect (chromosomal abnormality)
• poor sperm parameters (the presence of pathological forms of sperm)
• chromosomal abnormalities detected in a couple
• using sperm after TESE, MESA
• after treatment of oncological diseases using chemotherapy or radiation therapy and more …

Preimplantation genetic screening significantly increases the success rate of IVF treatment. In addition to couples with genetic risks, PGS can be used by all the other couples.

Examination process

For the examination we need to get the genetic material of the embryo by using a biopsy – a collection of a single cell or group of cells. Most often we do a cell biopsy of the embryos on day 3 or a biopsy of a group of cells on day 5 or 6 of the embryo development. This procedure allows an assessment of the complete genome of the future foetus (the maternal and paternal section). Currently, the trend is to investigate a more mature stage of the material development of the embryo, which brings more accurate information concerning the investigation of the embryo.

Can biopsy harm the embryo?

In the REPROMEDA centre, biopsies are performed by trained and experienced embryologists, and therefore the biopsy has no negative impact on the embryo’s ability. The collected cells are still undifferentiated, which is why their collection has no consequences in the form of damaging the foetus. The biopsy does not affect the genetic makeup of the foetus.

Which cell material is used for the examination?

The analysis most frequently uses:

• embryo cells – blastomeres
• groups of cells from the blastocyst (day 5 or 6) – i.e. trophectoderm
• pole bodies

At present, we do recommend the investigation of blastocyst – trophectoderm.

What methods are used for preimplantation screening?

REPROMEDA always tries to offer only the most modern methods. And for that reason, we have also begun offering, in addition to array-CGH (a-CGH) microarray technology, NGS (next generation sequencing). Both methods allow the testing of the entire spectrum of chromosomes, 23 pairs. NGS has even greater sensitivity and is even able to detect embryo mosaicism.

When to perform the embryo transfer?

In the case of an embryo biopsy on day 3, the embryos are further cultured until the fifth day. If the analysis shows that it is a suitable embryo without defects, the embryo is also transferred on the 5th day.
If cells are taken from embryos at day 5, the embryos are vitrified after the biopsy while the preimplantation examination is underway. A suitable embryo is then transferred in the next cycle. Freezing embryos is necessary because the endometrium is fully able to provide a good environment for the embryo implantation no later than the 5th or 6th day of the cycle.