What is PGS?
Pre-implantation Genetic Screening – Array CGH
The most common reason why IVF doesn't work is chromosomal abnormality – up to half of eggs in younger women, and up to 75% in women approaching 40 are chromosomally abnormal. The presence of an additional or absence of a whole or part of a chromosome is known as aneuploidy. The incidence of aneuploidy in eggs increases with a woman's age.
Now, for the first time in the UK, it is possible to test eggs and embryos to evaluate all their chromosomes in an IVF cycle, thus enabling the selection of the most viable embryos. We can achieve this by using the technique of Array CGH.
CGH stands for ‘Comparative Genomic Hybridization’, which is a complex molecular biology technique for the detection of chromosomal imbalances, particularly any gains or losses of DNA in the whole DNA of a cell. For IVF it is necessary that the sample used is no more than a single cell, hence it has taken many years – over a decade of research – to enable scientists to reach this point where we can measure changes in any of the 23 pairs of chromosomes from just a single cell.
CARE continued to develop the technology with the objective of providing all possible opportunities for our patients and the technique has been developed into a novel ‘Array CGH’ which has now been launched into clinical use for the first time.
Array CGH is a significant advance on conventional CGH as it is a more refined technique to detect changes in multiple copies of the chromosomes. The technique is also much quicker than conventional CGH, with results typically being available within 24 – 48 hours, compared with up to 5 days for conventional CGH. With our Array CGH technology we routinely get the results in time to transfer a fresh embryo, at the blastocyst stage.
How does Array CGH work?
Before a gamete (sperm or egg) matures and is capable of fusing with its counterpart, it must contain only half the number of chromosomes so that the newly formed entity (fertilized egg) has the full complement of 23 pairs. As gametes come from cells that during their development phase contain the full 46 (23 pairs) of chromosomes, they need to halve their chromosome complement before fusing together. The egg's ingenious way of doing this whilst, critically, still keeping most of its cell mass, is to shunt out in only a tiny amount of cytoplasm half of all the chromosomes; the tiny cytoplasm is called the ‘polar body’. The process of extruding the polar body gives us the opportunity to study the chromosomes of the egg without damaging the egg – by a process known as ‘polar body biopsy’. Once the polar body is removed it can be tested for its chromosome content using the Array CGH technology.
The technology of Array CGH has the potential to:
- Greatly improve IVF birth rate per embryo transferred.
- Minimize the incidence of miscarriages and birth defects caused by irregularity in the chromosome number of eggs.
- Reduce the incidence of multiple pregnancies whilst maintaining a high live birth rate.
