Pre-Implantation Genetic Diagnosis (PGD)
What is Pre-Implantation Genetic Diagnosis?
During your treatment with us, we may recommend Genetic Testing.
There are a number of significant inherited conditions, such as birth defects and genetic disorders that may concern potential parents when seeking In Vitro Fertilisation (IVF) treatment. Genetic Screening is amongst the newest and most advanced techniques used to test for genetic disorders. The method of Genetic Screening used in IVF is Pre-implantation Genetic Diagnosis (PGD).
PGD is generally used to detect irregularities and abnormalities in the number and / or structure of the chromosomes of embryos. This means we are looking directly at the DNA (i.e., the genetic material) contained in the developing embryo. In many cases, embryos with abnormal chromosomes may be unsuccessful during implantation to the uterine lining, leading to miscarriage, or the birth of a child with physical and/or mental disabilities.
PGD may help prevent adverse outcomes by identifying affected embryos as they are developing in the laboratory prior to being transferred to the uterus during an IVF cycle.
How can Pre-Implantation Genetic Diagnosis help us?
Patients may wish to consider PGD if there are concerns about:
- having a child with an inherited genetic condition,
- a family history of chromosome abnormality (on either side),
- previous chromosomal abnormality in pregnancy,
- advanced maternal age,
- recurrent miscarriage,
- repeated unsuccessful embryo transfers.
It is important that you understand all aspects of PGD prior to making the decision to undergo the procedure. The risks, benefits and alternatives of this testing will be discussed thoroughly with your treating team. This will include a genetic counsellor, your treating specialist and/or the scientist performing the procedures.
Why should we consider Pre-Implantation Genetic Diagnosis and Advanced Embryo Selection?
Patients may wish to:
- Reduce the incidence of miscarriage,
- Improve IVF success rates by selecting chromosomally “normal” embryos for transfer,
- Reduce the number of IVF cycles necessary to achieve a successful outcome, and
- Reduce the risk of a multiple pregnancy by transferring a single normal embryo.
In women aged over 35 years, approximately 35% of pregnancies are miscarried, with chromosomal issues accounting for more than 50% of these losses. By transferring only the chromosomally normal embryos, the number of successful pregnancies increases. Recent studies have shown a significant reduction in pregnancy losses after PGD, from 23% to 9%.
Who is Pre-Implantation Genetic Diagnosis recommended for?
We usually recommend PGD for patients:
- who have experienced recurrent miscarriage,
- who have experienced multiple previous unsuccessful IVF cycles,
- with a family history of chromosomal disorders,
- with an advanced maternal age, and / or
- wishing to increase their chances of a successful cycle.
An overview of the Pre-Implantation Genetic Diagnosis.
What do you test for?
All normal human cells contain 46 chromosomes in 23 pairs. We receive 23 chromosomes from each parent. The first 22 pairs of chromosomes are the same for men and women. The 23rd pair determines our sex: females have two ‘X’ chromosomes; males have an ‘X’ and a ‘Y’.
If the egg or sperm has an extra or missing chromosome, the resulting embryo will also have an extra or a missing chromosome. This is known as Aneuploidy. Having an extra chromosome is known as Trisomy (tri = three of a given chromosome) and having a chromosome missing is known as Monosomy (mono = one of the chromosome).
Aneuploidy in the larger chromosomes usually results in non-attachment of the embryo to the uterine wall or a lack of development soon after attachment. This results in miscarriage. If the Aneuploidy involves chromosomes such as the 13, 18, 21, X or Y, the pregnancy may continue with a chromosomal disorder. The most common of these is Trisomy 21 (known as Down Syndrome), XXY (known as Klinefelter Syndrome), Trisomy 13 (Patau Syndrome), and Trisomy 18 (Edward Syndrome).
While the characteristics of each condition depend upon the Aneuploidy, many include physical differences and mental retardation.
Unfortunately, the risk of Aneuploidy increases with advancing maternal age. Women are born with all the eggs they will ever have, meaning that eggs are as old as the woman. Over time, the chromosomes in the eggs are less likely to divide properly, resulting in the egg having an extra or missing chromosome.
The purpose of PGD for Aneuploidy is to select only chromosomally normal embryos for transfer to the uterus. The aim is to achieve more pregnancies, reduce the risk and number of miscarriages, and reduce the number of affected offspring.
How do you collect the material for testing?
There are two options currently available to collect genetic material for testing from embryos. Your treating team will decide which procedure to use and explain it to you prior to any collection taking place.
Polar Body Biopsy.
Each maturing egg produces two small cells, called the polar bodies, which degenerate after fertilisation. The chromosomal content of these cells allows us to deduce the chromosomal content of the egg itself.
In order to test the polar body, an opening is made in the outer covering of the egg and the polar body carefully removed and analysed. However, analysis of polar bodies provides information only from the mother. Chromosome abnormalities that may occur after fertilisation, when the sperm meets the egg, will not be detected via Polar body biopsy.
Embryo Biopsy or Blastomere Biopsy.
To test an embryo, some cells (called blastomeres) are removed via a microscopic opening made in the outer covering of the embryo during its fifth day of development (this is known as the “blastocyst” stage). The embryo is then frozen and kept in storage while the cells are analysed by PGD.
How do you perform the test?
There are two techniques currently available to analyse the cells collected.
Fluorescence In-Situ Hybridisation (FISH)
The Fluorescence In-Situ Hybridisation (FISH) technique uses probes, which have been labelled with different fluorescent dyes. These probes attach themselves to the chromosomes of the biopsied cells. The number of chromosomes of each type can then be counted. The genetic scientist can distinguish normal cells from cells with aneuploidy, but this technique can test a limited number of chromosomes.
Advanced Embryo Selection (AES)
Advanced Embryo Selection (AES) using array Comparative Genomic Hybridisation (CGH) analyses all chromosomes (including X and Y) compared to a sample of ‘normal’ DNA. A computerised scanner interprets the results, and these are then verified by a trained genetic scientist.
This technique provides very precise information and can detect chromosomal genetic disorders such as Down Syndrome and Edwards Syndrome.
PGD – What are the Potential Risks of Pre-Implantation Genetic Diagnosis?
- If an embryo is damaged by the biopsy, it may not be suitable for transfer into the uterus. The risk of damaging an embryo is less than 1%.
- Some biopsied cells may not give a test result, due to degraded DNA. The risk is less than 2% per cycle. Embryos without a result may still be transferred, but the advantages of PGD and Advanced Embryo Selection will not apply.
- The risk of a clinical misdiagnosis resulting in a foetus or baby with chromosome abnormalities after AES is less than 2%. We strongly advise prenatal testing by Chorionic Villus Sampling (CVS) or an amniocentesis because of this risk.
What are the alternatives?
The alternative to PGD is to undergo a cycle of In-Vitro Fertilisation (IVF) with no genetic testing of the embryos.
In this case, any resulting pregnancy will be recommended standard prenatal testing for abnormalities, including CVS, amniocentesis and ultrasound examination. These methods will identify some abnormalities in the foetus.