The Science Behind PGT (Preimplantation Genetic Testing): Understanding the Basics of Genetic Screening
These days, fertility experts have several methods to check for genetic conditions or chromosomal disorders in an embryo following fertility treatment like IVF, before implanting it back in your womb.
At BCRM, we’re dedicated to empowering you with the fertility knowledge you need to choose correctly for your body and family. In this guide, we learn about preimplantation genetic testing (PGT), the different types of PGT and how the process works.
What is genetic screening?
We have 46 chromosomes (or 23 pairs), which hold genetic information passed from your parents. The information held within your chromosomes dictates how you grow, develop and function.
However, abnormalities within your chromosomes can cause conditions such as Down’s and Edwards syndromes.
The PGT process analyses fertilised embryos for chromosomal disorders or abnormalities. Specialists can carry out PGT if you’re undergoing in vitro fertilisation (IVF) or intra-cytoplasmic sperm injection (ICSI). Depending on the type of PGT you choose, they can check for and rule out different conditions or disorders.
The genetic testing process
Once your egg has been fertilised with sperm outside the womb and the egg reaches the blastocyst stage, experts remove a small sample of cells (a biopsy). These cells are sent for a genetic analysis and the chromosomes are assessed using Next Generation Sequencing.
Experts can identify any whole missing (monosomies) or extra (trisomies) chromosomes to confirm or rule out potential chromosomal abnormalities.
Who is PGT for?
PGT may be beneficial if you have a history of recurrent miscarriages, several failed IVF attempts, previous pregnancies affected by chromosomal abnormalities or are over the age of 37. Chromosomally normal embryos are more likely to lead to a full-term pregnancy.
However, PGT does not guarantee pregnancy and you’ll still need to undergo regular scans and monitoring if you become pregnant.
What causes chromosomal abnormalities?
When sperm, egg or embryo cells divide, they should only divide into the right amount. However, sometimes this doesn’t happen. The cells can create too many or too few or miss chromosomes entirely.
Types of PGT
Your embryos can undergo three different types of PGT and each one can be tested for slightly different conditions or abnormalities. Your fertility specialist will discuss which is right for you so that you can feel confident in your genetic screening decision.
PGT-A (aneuploidy screening)
PGT-A is the standard genetic screening that checks for abnormalities within the embryo’s chromosomes. PGT-A doesn’t check for specific genetic disorders.
PGT-M (monogenic disorder screening)
PGT-M is where embryos are screened for specific genetic conditions. Currently, PGT-M can detect over 600 genetic conditions, such as cystic fibrosis, sickle cell disease and Huntington’s disease. This screening may be recommended if you have serious inherited conditions.
PGT-SR (structural rearrangement screening)
PGT-SR looks at the chromosomal structure of your embryos and checks for any segments that may be missing, duplicated or inverted. If you have a known chromosome structural rearrangement, this type of screening may suit you.
Are there any risks?
PGT carries some potential risks, but when you choose a reputable fertility specialist (like BCRM), the risk is minimised as much as possible.
Possible risks associated with PGT include:
Embryo damage – there is a less than 1% risk of an embryo becoming damaged and, therefore, not viable for transferring back into your womb
Risk of misdiagnosis – no test is 100% accurate, so there is a small risk of an embryo being misdiagnosed
What do my results mean?
When you undergo PGT, your specialist may give you one of three results:
Euploid – the embryo has a standard number of chromosomes and is suitable for the final step in your fertility treatment.
Aneuploid – the embryo carries an abnormal number of chromosomes, possibly monosomies or trisomies. Aneuploid embryos are not considered suitable for transferring back into your womb, as they have a higher risk of miscarriage and implantation failure.
Mosaic – the embryo has some cells with abnormal chromosomes and others with the usual amount. This can result in pregnancy, although live birth rates can be lower. Mosaicism can also lead to misdiagnosis as it can be unclear if the embryo has abnormalities.
Generally, you will have some embryos that are classed as euploid and some that are not.
However, there is a slight chance that none of the tested embryos are viable for transfer. If this
happens, your fertility specialist will discuss your options to further support you on your fertility journey.
How long do the results take?
It can take two to three weeks for your results to be ready. During this time, your blastocysts will be frozen to ensure they are protected and available for thawing should you proceed to implantation.
Embryo selection techniques
In the UK, PGT can only be used to screen for chromosomal abnormalities, not sex-selection purposes. It is illegal.
When you receive your PGT results, your fertility specialist will not know the embryos’ sex and the decision as to which embryos are viable for transfer will be based on the embryos’ assessment.
Discuss PGT with the Bristol Centre for Reproductive Medicine
PGT may help if your family history includes serious inherited conditions or if you’ve had recurrent miscarriages and failed IVF attempts.
At BCRM, we offer PGT for couples concerned about chromosomal abnormalities and disorders. We’ll discuss the possibility of PGT before you begin your fertility treatment so that you can be entirely prepared for the process.
Contact our team today to discuss your PGT options and how we can provide the support you need to continue or start your parenthood journey.