Pre-implantation Genetic Screening

Pre-implantation Genetic Screening is Compared to other mammals, human reproduction is extremely inefficient.

Pre-implantation Genetic Screening (PGS)

Preimplantation genetic screening (PGS), also known as preimplantation genetic testing for aneuploidy (PGT-A), determines the chromosomal status of an embryo by screening all 23 chromosome pairs, and tests for chromosomal abnormalities. PGS is used to genetically and chromosomally profile the embryo prior to implantation, so as to assess if the embryo has any defects or carries any congenital diseases.

Embryos with chromosomal abnormalities will not lead to a pregnancy, or will result in a miscarriage. The percentage of embryos that are abnormal can be affected by many factors, including the age and health history of the parents.

The main motivation for PGS is to find the embryo that is most likely to be successful since embryos with the normal number of chromosomes are most likely to implant and become an ongoing pregnancy. If an embryo with an abnormal number of chromosomes is used instead, it will most likely not implant at all or it will result in a miscarriage later. In some cases, embryos with chromosome changes will result in babies, but they would have conditions like Down or Turner syndrome, which PGS would have been able to detect.

How is it done?

  • Cell Retrieval - First, a couple of cells are extracted from each multi-cell embryo.
  • Cell Evaluation - Once several pairs of cells are gathered, each set is examined to gauge the potential presence of an abnormal gene in the corresponding embryo. Currently 4 technologies that can be utilised for assessment of normality of all 23 chromosomes Including Next Generation Sequencing (NGS), Array Comparative Genomic Hybridization (aCGH), Single nucleotide polymorphism microarrays (SNP), and Quantitative real time polymerase chain reaction (qPCR). Chromosomes are analysed to detect any duplications and/or deletions in the sequence.
  • Embryo Transfer - Following the selection of the best embryos from the cell evaluation routine, the embryologist will transfer up to two embryos to the uterus for implantation and successive pregnancy.

There are often embryos left over after the transfer to the uterus. The surplus embryos can be frozen using advanced cryopreservation facilities for use in future IVF cycles.

When is PGS used?

Usually pre-implantation genetic screening is employed in the following cases

  • Multiple in vitro fertilisation cycle failures
  • Recurrent miscarriages
  • If either of both partners suffers from congenital diseases such as thalassemia or colour blindness

Couples undergoing IVF may also select PGS because of severe male factor infertility, older age or a very large number of embryos. However, based on individual cases, the doctor takes a call on whether this technique is required for the couple. PGS also provides valuable insights to researchers studying IVF.