Latest update: 11/05/2017


In PGS or pre-implantation genetic screening we count the number of all chromosome pairs in one or several cells of the embryo. Embryos with an abnormal number of chromosomes are termed aneuploid, hence this technique is also called ‘screening for aneuploidy’. Aneuploid embryos are not returned to the uterus because they are not viable or can give rise to the birth of a child with problems.

Difference between PGD and PGS

  • In PGD we are looking for one particular hereditary gene defect: in the lab we already know what defect on what chromosome or gene we are looking for.
  • In PGS by contrast we are not evaluating one particular gene or chromosome, rather we count the chromosomes in the cells that we have obtained from the embryo.
    Thanks to PGS we can make a selection in the lab of fertilised embryos, not only based on their morphology (the structure and the number of cells), but also on their chromosomal contents.


Genetic embryo analysis and embryo transfer

Under PGD we have seen that there are two options in timing the embryo biopsy and the embryo transfer.
  • In PGS we prefer to perform the genetic analysis on day five after fertilisation, because then we can examine several cells and thus more DNA.
  • It is also possible that we perform a biopsy at day 3, with the same microarrays, and that we freeze the embryos at day 5 of development.

In short, in all cases the transfer back of the embryos with normal chromosomes is scheduled for a later, non-stimulated cycle – and after the genetics doctor has discussed the result of the screening with you.

Nevertheless, if only a limited number of embryos are available at day three of development, it may be possible – after discussing it with your doctor – not to perform genetic screening and simply return the available embryos to the uterus.