PGS (Pre-implantation Genetic Screening)

PGS (Pre-implantation Genetic Screening)


Pre-implantation genetic screening, PGS, in short, evaluates the status of the embryo by considering the chromosomal makeup. Abnormalities in the chromosomal makeup of the embryo might cause a specific syndrome to occur in the offspring or may cause a miscarriage. The test is conducted as part of the in vitro fertilization that can treat the infertile couples. The embryos are tested for the presence or absence of certain chromosomal abnormalities before being transferred into the uterus.

When is PGS appropriate?

The IVF-PGS would generally be performed at a very early stage of embryo development. A biopsy is generally performed on day 5 on embryo development. The comparative genomic hybridization (CGH) can evaluate all the chromosomes. This, coupled with the better freezing techniques that utilize vitrification, offers promising results. PGS is considered in the following cases:

  • Older women who have an increased risk of having a baby with a chromosomal problem, like Down's syndrome
  • A history of recurrent miscarriage
  • Women who have had a previous pregnancy with chromosomal problems
  • Repeated failures in IVF

How is PGS performed?

There is no change in the initial part of the IVF is PGS is to be performed. The five basic steps include:

  • The eggs are allowed to ripen in the ovaries
  • The eggs are then retrieved
  • Fertilization of the eggs take place and the embryo formed starts to grow
  • A biopsy is conducted on the embryos and comparative genetic hybridization screens the embryos for chromosomal abnormalities
  • The embryos are frozen and the test results are achieved and unaffected embryos are transferred in a frozen embryo cycle

How safe and accurate is PGS?

If the well-being of the embryo is being determined in a Day 5 biopsy on the basis of evaluating the chromosomes in a few cells of the embryo that contain at least a hundred cells, the estimated diagnosis rates with the help of CGH is quite low. This is in consideration of the fact that the results are quite rapid.

  • The misdiagnosis might occur if the cells that are removed do not represent the major cell line in the embryo. Not all the cells in the early embryo would be identical, which is known as mosaicism.
  • It is not possible to discern through CGH if there are extra copies of all chromosomes.
  • It would also not be able to discern a balanced translocation.
  • Another obvious concern is the possible injury to the embryo during the biopsy would lower the success rates. The chances reduce with the day 5 biopsy than with the day 3 biopsies.

What are the possible results from the PGS?

There could be lots of possible results from the PGS:

  • Normal embryo - the number of chromosomes are normal and suitable for transfer
  • Abnormal embryo - there is significant losses or deletions in the chromosomes and cannot be transferred
  • Partial deletions - a small amount of genetic information is missing from a few of the chromosomes, contributing to lower chances of pregnancy
  • Mosaic - a mixture of normal and abnormal cells within the analysed cells
  • No result - in approximately 5% of the cases, the genetic information might be possible to be analysed in the lab

Is there an alternative to PGS?

The alternative is to go through conventional fertility treatment. Prenatal diagnosis through chorionic villus sampling or amniocentesis using molecular diagnostic techniques would be the alternatives. More material could be sampled from the pregnancy and it would take more time for interpretation. However, the chances of misdiagnosis could decrease. However, the only options to the couple would include giving birth to a child with a defect or terminating the pregnancy.

Your doctor would be able to guide you on the available options.

Authored by Dr. Radhika Meka

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