Pre-implantation Genetic Screening
For over twenty years, it has been possible to test embryos for chromosomal abnormalities during an IVF cycle to identify the embryos with the best chance of developing into healthy babies. This process is called Pre-implantation Genetic Screening (PGS). The technique used until recently for PGS is fluorescent-in-situ hybridisation (FISH). Unfortunately, with FISH it was possible to test only a fraction of the chromosomes (up to 10 out of the 23). It has also been shown to sometimes deliver inaccurate results. A new technique called PGT-A is now available to test all 23 chromosomes in a single cell or blastomere.
PGT-A
Up to 40-80% of human embryos may contain abnormal number of chromosomes (known as ‘aneuploidy’), depending on the woman’s age. Most chromosomally abnormal embryos will not implant and if they start to implant are most likely to miscarry. PGT-A makes it possible to screen eggs or embryos so as to select only those chromosomally normal for transfer into the woman’s uterus, thereby potentially improving the chances of successful implantation and reducing the risk of miscarriage.
Who is it for?
PGT-A may be particularly beneficial for women over 35 years of age, couples experiencing recurrent miscarriage or recurrent IVF failure, or women who have had termination of one or more chromosomally abnormal pregnancies.
Preimplantation genetic diagnosis (PGD)
PGD is a laboratory test, performed on eggs or embryos during an IVF cycle, that enables couples to check whether their pregnancy may be affected by a chromosomal disorder or an adverse genetic condition. It can be used to screen for many genetic conditions, including single gene disorders and chromosomal abnormalities. Many couples ask for PGD to test for aneuploidy (such as Down syndrome). It is thought that a large proportion of early miscarriages may be due to aneuploidy, which increases with the age of the female partner. Others wish to test for specific single gene disorders or genetic conditions that run in the family, such as cystic fibrosis or muscular dystrophy. It can also be used to screen eggs and embryos of individuals who carry a chromosomal translocation, significantly decreasing the risk of miscarriage or birth defects.
HFEA comment
Are there any risks associated with PGT-A?
There is a possible but unquantified risk that removing cells from an embryo may damage it and prevent it from developing. However, the first births following embryo biopsy for genetic screening were reported in 1990 and since then, thousands of children have been born, and health risks following embryo biopsy have not been identified as different from IVF/ICSI pregnancies in general.
The necessary freezing and thawing of embryos is highly successful, with a survival rate of over 95% after thawing. There is, however, a small risk that the embryo will not survive the process.
What is the evidence for PGT-A?
The independent regulator of fertility treatment, the Human Fertilisation and Embryology Authority (HFEA), has developed a ‘red-amber-green’ rating system and provides information on their website about treatments that are offered on top of your routine fertility treatment – known as treatment add-ons. They consider that the only way to be confident that a treatment is effective enough to be used routinely is to carry out a randomised controlled trial (RCT). In an RCT, patients are assigned randomly to two groups: a treatment group, given the new treatment and a control group, given a well-tried treatment or a placebo.
The HFEA has given PGT-A a red rating because it considers that there is no evidence from such trials that PGT-A increases the overall chances of having a baby. There is information on the HFEA website about PGT-A that you may find useful to consider.
The Zita West Clinic is part of the CARE Fertility group, and CARE did participate in a large scale RCT called the STAR trial, which was published in December 2019. This trial concluded that there was a significant increase in pregnancy rate in a subset of women aged 35–40 where they had more than two embryos available for testing, although overall in all age groups there was no increase in ongoing pregnancy. However, we recognise that that more robust clinical and laboratory trials are needed to prove whether or not PGT-A significantly increases live birth rates.
It is important to understand that PGT-A will not increase your overall chances of having a baby, although it may shorten your time to pregnancy by helping us select the best embryo sooner. For more information, please speak with our team, who will give you detailed patient information documents which outline all the relevant points about PGT-A, or you can visit the HFEA website.
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