Pre-implantation Genetic Screening
Array CGH
Our Nottingham and Manchester clinics both provide Chromosome Screening Programmes. A satellite service is also available through our Sheffield and Northampton clinics.
Currently CARE uses two distinct approaches to chromosome screening:
- Eggs - testing of the chromosomes in the polar bodies of the egg
- Embryos - testing of the chromosomes of a Blastocyst, by removal of Trophoblast cells
Chromosome screening is performed using one of two methods:
- Array CGH (Comparative Genomic Hybridisation)
- Blastocyst chromosome screening with Parental Support R technology
Approximately 70% of embryos produced either through natural conception or IVF are lost before birth and most before implantation. A major cause of embryo loss, including miscarriage, is a chromosome anomaly (known as 'aneuploidy'), where there is either a loss or a gain of one or multiple chromosomes.
CGH stands for Comparative Genomic Hybridization and Array CGH is the latest approach of this technology that examines the entire number of chromosomes in a single cell to check for chromosomal imbalances, particularly any gains or losses of DNA in the whole DNA of a cell.
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. It is also much quicker and results are typically available within 24 - 48 hours allowing a fresh embryo transfer, whereas conventional CGH can take up to 5 days, making it necessary to freeze the embryos and transfer at a later date.
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.
Blastocyst Chromosome Screening (BCS)
Blastocyst Chromosome Screening now enables us to extend our range of chromosome screening options available to patients. It involves the analysis of the total chromosome complement of a single cell or group of cells from each embryo following IVF treatment. Only those embryos having cells with chromosomally normal results are placed in the womb.
This test may benefit a certain group of patients, over and above our regular 'egg' screening.
Advantages of Blastocyst Chromosome Screening
- Information of the whole chromosome complement of the embryo
- Therefore sperm chromosome 'contribution' obtained, up and above only egg analysis
- Post fertilisation spontaneous abnormalities recognized
- Clinical ongoing pregnancy rates at 8 weeks: current data = 73% per embryo transfer
- Aiming for single blastocyst transfer to achieve 'one healthy pregnancy' per cycle
