Pre-implantation genetic testing
Pre-implantation genetic testing (PGT)
Pre-implantation genetic testing (PGT) is a tool that is being used more and more lately.
PGT tests the embryos created through IVF before embryo transfer.
There are two types of PGT:
- Pre-implantation genetic diagnosis (PGD) is used to diagnose specific gene abnormalities
PGD can be used when one or both parents have a known genetic disorder and they do not want to pass it on to the child. The aim is to identify only the healthy embryos that are free of the disease (e.g. cystic fibrosis, thalassaemia) and make them available for embryo transfer.
This method tests for specific diseases and does not exclude the possibility of a child being born with a completely different genetic impairment.
PGD can be used even if the couple have no fertility problems to ensure that their baby will not inherit specific genetic disorders.
- Pre-implantation genetic screening (PGS) is used to determine if an embryo has normal karyotype
On average, 2 out of every 3 embryos from women aged 35-40 have chromosomal abnormalities. This percentage is further increased (4 out of every 5 embryos) in women over 40.
PGS tests the embryos available for any chromosomal abnormalities so that only chromosomally normal embryos are transferred back to the womb.
Both of these methods are performed only in the context of IVF and are usually applied either on the 3rd day or on the 5th day (preferably) of embryo culture.
The embryologist takes a small biopsy out of the embryo and this is tested.
Children born after IVF and PGT are just as healthy as children born after IVF only.
In which cases may PGT be considered?
- Couples with a burdened genetic history
- Abnormal karyotype of the woman or the man
- Repeated unsuccessful IVF attempts
- Recurrent first trimester miscarriages
- Severe male infertility
- History of a child born with chromosomal abnormalities
- Female age over 40
Are there any disadvantages to PGT?
- PGT results may be misleading. Because of the possibility of mosaicism, it is possible that the biopsy taken from the embryo may not be representative of the whole and give the false impression that the embryo is abnormal, when in fact this would not be the case (sometimes the opposite may be true; i.e. the embryo may be thought to be normal when it is in fact abnormal)
- PGT does not guarantee 100% accuracy in the results. Checking a small portion of the fetus before implantation is not the same as checking the fetus itself in advanced pregnancy. This is why it is often recommended to perform prenatal screening on pregnancies that result following PGT.
- The procedure of PGT is invasive and may damage the embryo and therefore reduce its chance of implantation (even if the embryo is normal)
- There is always the possibility of the embryo self-healing even if it is abnormal to start with; this will not happen if the embryo is discarded due to being considered ‘abnormal’. Therefore, even if we are certain of an embryo being abnormal, there is no way we can predict whether it can self-heal later on and give rise to a normal pregnancy
- PGT is associated with significant extra cost