Comparative Genomic Hybridisation (CGH) screening
CGH is an emerging form of preimplantation genetic screening (PGS) that is used to detect abnormalities in the number of chromosomes in an egg or an embryo. Because this is a new form of screening and has only been used in a small number of cases, it is not yet possible to say how successful it is.
*London Fertility Centre is one of the very few centres in the UK to offer this to patients as well as PGS and PGD
With ‘standard’ PGS, which uses a technique called fluorescence in situ hybridisation (FiSH), even the most advanced labs can only test up to 12 chromosome pairs, not the full 23 pairs tested using CGH.
CGH takes advantage of new ‘microarray’ technology that provides much more detail than ‘standard’ PGS. The full set of 23 pairs of chromosomes can be compared so that abnormalities in the numbers of multiple chromosomes can be detected. Find out more from HFEA
Preimplantation Genetic Diagnosis (PGD)
PGD is an early prenatal diagnosis, designed for couples at risk of transmitting inherited disease to their children because of chromosomal abnormalities. The major advantage of PGD is that it allows couples to identify embryos that may be carrying genetic disorders and avoid the difficult choice of abortion, or possible miscarriage, during the pregnancy. Structural chromosomal abnormalities can include a number of different chromosomal defects and usually occur when there is an error in cell division.
Patients having PGD will undergo routine IVF to generate multiple embryos to increase the chance of obtaining chromosomally normal embryos. Please refer to our IVF Patient Information Sheet for more information about IVF.
Both partners will have a cytogenetic karyotype test and this can be carried out at LFC if it has not been done elsewhere. Our laboratory will then analyse your case over a period of two to three months, and both partners will meet with our genetic counsellor before commencing a typical IVF cycle.
Once the embryos are created, our laboratory will perform an embryo biopsy procedure using state-of-the-art laser drilling to create access for the removal of single cells from each embryo for testing. Laser drilling allows for higher accuracy cutting when accessing cells from within the delicate embryos. Following a diagnosis period of 48 hours, approved embryos (up to two) are then transferred into the womb.
Preimplantation Genetic Screening (PGS) with Comparative Genomic Hybridisation (CGH)
Preimplantation genetic diagnosis (PGD) is an early method of prenatal diagnosis, designed for couples at risk of transmitting inherited disease to their children. More recently, the PGD procedure has been used to help specific groups of infertile patients including older women, those with recurrent IVF failure, or unexplained recurrent miscarriages, as women from these groups are at greater risk of producing embryos with abnormal chromosomes (aneuploid embryos). This modified PGD procedure is called Preimplantation Genetic Screening (PGS).
Previous PGS tests only allowed a limited number of chromosomes to be examined in each embryo, however the Comparative Genomic Hybridisation (CGH) method used at LFC is one of the newest PGS methods, and increases accuracy through analysis of all chromosomes in the embryo to determine how many copies there are.
There should be two copies of each chromosome in a normal embryo. Having extra or missing chromosomes (aneuploidy) can result in failure of an embryo to implant, as well as miscarriage and conditions such as Down’s syndrome.
What are the benefits of PGS?
The aim of PGS is to increase the chances of identifying embryos that contain the correct number of chromosomes. The benefits of doing this is that it helps embryologists to identify the embryos most likely to produce a pregnancy, allowing them to be prioritised for transfer to the womb, thereby improving implantation rates in this select group of patients and reducing the risk of miscarriage. PGS does not, however, guarantee the birth of a baby without abnormalities, chromosomal or otherwise.
PGS patients will undergo routine IVF to generate multiple embryos to increase the chances of obtaining chromosomally normal embryos.
PGS screened embryos can also be frozen for transfer later. Our data has shown that the technique of vitrification to cryopreserve embryos is very reliable and has a near 90% survival rate.
What happens if none of the embryos are chromosomally normal?
If no embryos are found to be chromosomally normal, the embryo transfer may not take place. The likelihood that this will happen is influenced by a variety of factors, the most important of which is age and number of eggs retrieved.
For female patients over 40 years of age around 13% of cycles do not have any embryos transferred due to all embryos being abnormal. For younger patients the risk of all embryos being abnormal is less.