Preimplantation genetic testing: an overview

To help avoid this confusion and facilitate communication between fertility specialists, researchers and patients, a group of reproductive health organisations developed a standardised glossary of fertility-related terms and definitions.

This glossary includes recommendations for the terminology used to describe genetic testing of embryos. Previously referred to as PGS (preimplantation genetic screening) and PGD (preimplantation genetic diagnosis), all genetic testing on embryos is now known as PGT (preimplantation genetic testing).

The different types of genetic testing are differentiated by the addition of further letters to the PGT acronym: namely PGT-A, PGT-M and PGT-SR. To understand the meaning of these terms and the types of genetic testing they refer to, it first helps to understand some basics of genetics.

A brief lesson on genetics

You may have heard of the term ‘chromosome’ before. Chromosomes are structures that house our DNA, which is the genetic code that contains all the instructions required to make you, you! Normally, every cell in the body contains 46 chromosomes. These chromosomes are inherited – during fertilisation (when egg meets sperm), 23 chromosomes are passed on from each parent.

There are many different types of genetic errors that can occur, and a wide range of genetic conditions they can give rise to. One type of error is an abnormality in the number of chromosomes, which is referred to as aneuploidy. Instead of 46 chromosomes, a person may have 45 or 47 chromosomes, as seen in Turner and Down syndrome, respectively. Embryos that have an abnormal number of chromosomes are less likely to result in an ongoing pregnancy.

Small errors may also occur in the DNA contained within the chromosomes. These errors are known as genetic mutations and can give rise to diseases such as cystic fibrosis and haemophilia.

Lastly, errors can occur in the structure of the chromosomes, including changes to their size or how the DNA is arranged within them. This is not the same as aneuploidy – many people with structural rearrangements have a normal amount of DNA overall. If that is the case, the person will probably not be affected by the rearrangement. However, someone with a structural rearrangement has an increased risk of producing an embryo with an abnormal amount of genetic material, which is less likely to result in a healthy, ongoing pregnancy.

The different types of genetic testing

There are three different types of genetic testing that may be performed on embryos:

• PGT-A (preimplantation genetic testing for aneuploidies): PGT-A used to be known as PGS. It is designed to identify the embryos that have an abnormal number of chromosomes (aneuploid embryos), as these embryos are less likely to result in an ongoing healthy pregnancy.
• PGT-M (preimplantation genetic testing for monogenic or single gene defects): PGT-M was previously known as PGD. PGT-M assesses the embryo’s genetic code to determine if it contains mutations that may result in certain genetic conditions, such as cystic fibrosis or haemophilia.
• PGT-SR (preimplantation genetic testing for chromosomal structural rearrangements): Like PGT-M, PGT-SR was also previously known as PGD. PGT-SR detects structural rearrangements in an embryo’s DNA and can detect whether the embryo has the correct amount of genetic material.

How do you know if genetic testing is appropriate for you?

Not all patients require PGT, and each type of PGT is only appropriate for particular groups of people. Your Newlife IVF fertility specialist can advise whether genetic testing is suitable for you. To make an appointment, call (03) 8080 8933 or book online via our appointments page.