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.
For which chromosomes can we test today?
A standard technique – fluorescence in situ hybridisation or FISH – works with coloured DNA markers and enables the analysis of chromosomes 13, 16, 18, 21, 22, X and Y.
These chromosomes often lie at the root of miscarriages or are involved in a number of severe abnormalities such as trisomy 21 (Down’s syndrome).
A more recent technique uses microarrays and enables us to count all the chromosomes. At this point in time we nearly always use this technique for PGS.
A DNA microarray (also commonly known as DNA chip or biochip) is a collection of microscopic DNA spots attached to a solid surface (in glass or plastic). The technique allows us to analyse the amount of DNA present in an embryo biopt and to deduce whether the sample contains a normal count of chromosome.