Preimplantation Genetic Testing (PGT)
Key Facts
- PGT-A is a reproductive technique used at the same time as IVF.
- We use PGT-A to choose embryos with a normal chromosome number.
- PGT-A is a screening test that isn’t 100% accurate. It does not tell us about any other genetic conditions.
What are chromosomes?
Human genetic information is bundled into DNA strings called chromosomes.
A normal embryo contains 23 pairs of chromosomes. Each pair has one chromosome from the mother’s egg and one from the father’s sperm. It is a euploid embryo if there are 46 chromosomes paired into 23 pairs.
Each chromosome pair is labelled 1 to 22 with the 23rd pair being the sex chromosomes (X and Y).
What is aneuploidy?
Aneuploidy is where a chromosome pair has an extra copy or a missing copy of a chromosome rather than the usual two. This is due to random errors in division of these chromosomes.
Aneuploidy can lead to:
- a potential miscarriage,
- failure of the embryo to implant into your uterus or
- a baby being born with a condition. One example of this is Down syndrome where the embryo has an extra chromosome 21.
With PGT-A, we can detect this error in chromosome number. This is why it is also known as 24 chromosome screening.
Who is PGT-A suitable for?
- People with a history of recurrent implantation failure
- Women who are aged over35 years and are expected to develop a good number of blastocysts during IVF treatment. PGT-A may lead to a shorter time to an ongoing pregnancy.
What happens in PGT-A?
Preparation
- You will have a medical consultation to talk about if PGT-A IVF is right for you
- After egg collection and insemination, the embryos are grown in the lab for 5-7 days.
Testing your embryos
- We biopsy an embryo. This biopsy involves taking a few cells from the trophectoderm of the embryo. The trophectoderm is the outer cell layer of the embryo that later develops into the placenta. Studies have shown that in most cases, the development of the embryo isn’t affected.
- These cells are then sent to a lab specialising in DNA testing.
- The embryos are frozen while we wait for the results, which take up to 6 weeks.
Transferring the embryo
- If your results show that you have a euploid embryo, you may have a frozen embryo transfer.
- Unused normal embryos will remain in storage.
- We will talk to you about what you would like to do with embryos with abnormal results.
What are the limitations of PGT-A?
Embyros
- You may not have blastocysts that are suitable to have outer cells removed (biopsy).
- You may have no euploid embryos available for transfer.
- Some euploid embryos may not survive being frozen and thawed.
- There is a possibility of a mosaic result. This is when some cells in the embryo have a different number of chromosomes compared to other cells. If this happens, it does not necessary mean that embryos cannot be transferred. You would need to have a further detailed talk with a genetic counsellor.
- Sometimes there is no result for an embryo. You may choose to transfer this embryo despite not knowing the results.
Limited testing
- There maybe other genetic conditions or birth defects present which PGT-A does not test for.
- PGT-A is not 100% accurate. The accuracy is shown to vary from 90-99% depending on the test type that is used.
- We still recommend routine prenatal screening after PGT-A.
- PGT-A cannot be used for sex selection of the embryo unless there is a pre-existing sex-linked condition in the parents. This must be approved by an ethics committee.
What are the costs?
PGT-A has an additional cost to that of an IVF cycle. Please ask your doctor/nurse for the price list.
How do I arrange PGT-A with the laboratory?
If you have been considering PGT-A, you will need to let the laboratory know you wish to proceed. You must inform us at Book on in order to be booked in with the lab. The best way to communicate about PGT-A is by email fertlab@adhb.govt.nz.
If you have not let us know, your IVF cycle may proceed as normal but without PGT-A.