PGT-A Results Explained
PGT-A checks all 23 pairs of chromosomes to determine whether embryos are abnormal (aneuploid), normal (euploid), or somewhere in between.
What is PGT-A?
Preimplantation genetic testing for aneuploidy (PGT-A) is an optional add-on treatment which is becoming increasing popular in IVF. It tests whether embryos have the correct number of chromosomes which helps to identify the ones that are most likely to be successful and eliminates embryos which would never be able to make a healthy pregnancy. Chromosome abnormalities are surprisingly common and they are thought to be the biggest cause of failure in IVF as well as a leading cause of miscarriage.
A healthy embryo should have 23 pairs of chromosomes – one set from the egg and one set from the sperm. 22 of the pairs are simply numbered 1-22 and 1 pair are the sex chromosomes; either XX in female embryos or XY in male embryos. However, genetic errors in eggs and sperm are common and this means that embryos often have extra or missing chromosomes (known as aneuploidy). PGT-A checks all 23 pairs of chromosomes to determine whether embryos are abnormal (aneuploid) or normal (euploid).
A euploid result means the embryo is normal. The cells that were tested have the correct number of chromosomes so the embryo is safe to transfer with approximately a 70% chance of making a pregnancy. One study found that patients with 3 euploid embryos had 94.9% chance of achieving a pregnancy.
An aneuploid result means the embryo is chromosomally abnormal and is not safe to be transferred. The cells that were tested may have extra or missing whole chromosomes or pieces of chromosomes. The results report may specify which chromosomes are affected for each embryo and what type of abnormality is present, some of which are listed below. Rest assured chromosome errors are usually just random and it doesn’t mean you have any genetic condition.
- Monosomy – missing chromosome
- Trisomy – extra chromosome
- Deletion – missing piece of chromosome
- Duplication – extra piece of chromosome
- Polyploid – at least one extra copy of every chromosome
A mosaic result means that a mixture of euploid (normal) and aneuploid (abnormal) cells were found. For example, if 5 cells were taken from the embryo and 1 cell was abnormal but the other 4 were normal, the embryo would be reported as mosaic. This would be a low level mosaic since 80% of the cells are normal so the embryo is more likely to be healthy overall and is usually still suitable for transfer. However if 4 of the cells were abnormal and only 1 was normal, the embryo would be a high level mosaic and would most likely be unsuitable for transfer. It also depends on which chromosomes are affected. Your doctor, embryologist or genetic counsellor will be able to guide you as to whether the embryo is suitable for transfer.
In around 5% of cases, it is not possible to diagnose an embryo from the biopsy taken. This is most commonly because too few cells were removed from the embryo so there is not enough DNA to test, the sample was contaminated, or the sample didn’t meet the quality control parameters to give a reliable result.
No result embryos can be still be transferred and they have the same chance of success as untested embryos. It may also be possible to retest the embryo by thawing it, biopsying it again and re-freezing it. This depends on what quality the embryo was initially – some embryos are not able to survive a second biopsy.
Are the results reliable?
The technology used to analyse an embryo’s DNA is relatively new but it has become incredibly advanced in a short space of time. There is no doubt that it can provide valuable information about the embryo’s genetic health which can be very useful in selecting embryos for transfer. However, there is still lot of uncertainty about the reliability of the results. It is not uncommon to miss an abnormality or to wrongly detect one that isn’t actually there. It is important to remember that only a small number of cells are taken at random out of an embryo that contains hundreds of cells, so it is unlikely that this small sample can give a definitive answer. For example, it may be that the majority of the embryo is normal but we happened to sample some abnormal cells, or vice versa. Furthermore, a biopsy is only taken from the outer portion of the embryo known as the trophectoderm which forms the placenta and not the actual baby. There is some uncertainty about whether these trophectoderm cells are representative of the whole embryo or would we get a different result if we have sampled cells from the part of the embryo which forms the baby?
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