During fertilisation, i.e. when sperm and egg come together, the mother and father pass on 23 chromosomes each – so that the embryo that forms has a total of 46 chromosomes. Chromosomes are rope-like structures made up of tightly-spiralled DNA. You should think of DNA like a line of code and different sections of this code form genes – the genetic blueprint responsible for making you unique.
In addition, humans also have special sex chromosomes responsible for determining an individual’s gender. For instance, biologically female individuals have two X chromosomes (XX) while those who are biologically male have one X and one Y chromosome (XY).
Sometimes when an egg and sperm come together, genetic errors occur. If the pregnancy is successful, these errors may lead to a baby being born with a genetic condition. For example, if the chromosomes from the mother and father fail to combine correctly, the embryo may have an abnormal number or combination of chromosomes (called aneuploidy). One of the most familiar aneuploidies is Down Syndrome, whereby a baby is born with an extra copy of chromosome 21.
Changes or genetic mutations can also occur to the DNA (code) that makes up a chromosome. Genetic mutations within your DNA can give rise to faulty gene copies with the potential to cause an inherited medical condition. Cystic fibrosis and spinal muscular atrophy are two well-known examples of rare hereditary diseases. In some families, one or both parents, or even another relative might show symptoms of a particular condition and thus already know that they have a chance of passing on a particular condition to any offspring. However, other parents can be carriers of a faulty gene without knowing or showing any symptoms. Depending on how the condition’s pattern of inheritance works, one or both parents may need to pass on the faulty gene for the child to manifest the disease.
Changes in the structure of a chromosome can also lead to a genetic error. This includes alterations to the size of the chromosome or the arrangement of the DNA. Structural abnormalities within chromosomes can give rise to embryos with irregular amounts of genetic material. Typically, implanting these embryos is less likely to result in a successful pregnancy, due to failed implantation or miscarriage.
There are different types of genetic screening that we can perform before and during IVF. Deciding if testing is appropriate, and which test is best for you, will depend on your situation and personal wishes. Below is a description of the different types of genetic tests available.
As mentioned above, a carrier is an individual who has no symptoms of a disease but has a faulty gene that they could pass onto their unborn child. If a couple wants to determine their risk of passing a condition on to their offspring, they can undergo a simple blood or saliva test prior to conception to check if they are a carrier of any rare diseases. If you decide that carrier screening is right for you, you and/or your partner can choose to test if you are carriers for a single specific condition, a group of common conditions, or even hundreds of conditions (expanded carrier screening).
Single-condition screening identifies changes in one specific gene. It is usually offered to individuals who have a family history of a known genetic condition (e.g. cystic fibrosis) or are from a particular ethnic background considered high risk for certain inherited disorders (for example, certain rare disorders such as Tay-Sachs disease occur more often in people of Eastern European (Ashkenazi) Jewish heritage than in the general population). Even without a personal or family history of a genetic condition, some people choose to reduce their risk by testing for the more common rare hereditary diseases, such as cystic fibrosis, fragile X syndrome and spinal muscular atrophy. In expanded carrier screening, people test not only for these three conditions, but also faulty genes linked to many hundreds of other rare inherited conditions.1
If either partner is identified as a carrier of a particular disease, the couple may opt to go down the path of IVF, so they can employ preimplantation testing of their embryos. By doing so, they can ensure that only a healthy embryo is transferred into the woman’s womb, bypassing the risk of inherited disease associated with natural conception.
Before implanting an embryo during IVF, we can test your embryos to determine which has the more normal genetic material and therefore the best chance of resulting in a healthy pregnancy. This process is called preimplantation genetic testing (PGT) or embryo screening.
PGT involves taking 5–7 cells from the outer layer of an embryo. These cells would normally go on to form the placenta and not the baby itself. While embryos are fragile, the risk of harming the embryo during this process is minimal, so long as the biopsy is performed by an experienced embryologist in a high-quality lab. From the sample collected, we can identify if a genetic error is present in the embryo.
There are three different types of PGT.
PGT-A stands for preimplantation genetic testing for aneuploidies (you may have previously known this test as preimplantation genetic screening or PGS for short). PGT-A assesses embryo health by checking for the normal number of chromosomes. It can increase the chance of pregnancy and lower the chance of miscarriage by ensuring that only genetically healthy embryos are transferred into the uterus. This type of testing can be especially useful for older women (>37 years), whose embryos are more likely to have genetic errors, and for women experiencing recurrent miscarriages or repeated unsuccessful IVF cycles. It may be also be requested by women who have a family history of chromosomal disorders or a previous Down syndrome-affected pregnancy.
PGT-M or preimplantation genetic testing for monogenic or single-gene defects is used to assess whether specific mutations are present in an embryo’s genetic code. This type of testing may be requested if a certain condition is known to run in the family or either parent is a known carrier of a disease. In this case, PGT-M is used to test if a specific disorder is present in an embryo. The purpose of testing is to prevent the parents passing on a hereditary condition by only transferring embryos that don’t carry the gene for that disease. For example, PGT-M can determine if the embryo’s DNA contains changes that could lead to haemophilia or cystic fibrosis.
PGT-SR, short for preimplantation genetic testing for chromosome structural rearrangements, identifies if there are any structural rearrangements in an embryo’s DNA. PGT-SR also allows us to determine whether there is a normal amount of genetic material present in the embryo. We may recommend PGT-SR if you or your partner carry a chromosomal rearrangement. We may also recommend PGT-SR if you have previously had a child or pregnancy affected by a chromosomal rearrangement.
In good news, from 1 November 2021, PGT and related embryo biopsies will be added to the Medicare Benefits Schedule. This means that genetic screening of embryos will now be subsidised for Australians who are known carriers of a genetic condition, to identify if any of their embryos have inherited the genetic mutation/chromosomal abnormality for that condition and are at risk of being born with the disorder. The Medicare rebate will be restricted to one PGT test per embryo produced during a single Assisted Reproductive Treatment (ART) cycle. To be eligible for the rebate, these genetic tests must be requested by your fertility specialist, not your GP.
As PGT-A involves taking an in-depth look at the chromosomes in an embryo, it can also identify the presence of sex chromosomes, and therefore gender. In Australia, it is illegal to select an embryo for implantation during IVF based on gender for a non-medical purpose – i.e. you can’t pick an embryo because it will result in a boy or girl.
As per the National Health and Medical Research Council ethical guidelines, “PGT may only be used to select against genetic conditions, diseases or abnormalities that would severely limit the quality of life of the person who would be born.”2 Therefore, if the sex of an embryo is associated with a particular genetic condition, sex selection via PGT-A may be acceptable. Typically, the part of the test results that help determine the embryo’s sex is not revealed unless there is a risk of passing on a genetically abnormal embryo.
A genetic counsellor can discuss with you the various considerations before and after any form of genetic testing. This may include the possibility of PGT detecting a genetic condition that you and/or your partner were not specifically screening for. Genetic counsellors can also provide information about how a genetic condition could impact the person and their families, as well as what types of therapies are currently available to reduce the severity of the condition.
It’s important to note that these types of tests are not automatically performed during IVF and are not appropriate for all patients. However, in the right circumstances they can represent valuable tools for preventing serious hereditary diseases and aiding IVF success. Deciding whether a particular type of genetic screening is right for you will depend on your situation. Your Newlife IVF fertility specialist can help you decide whether genetic testing is appropriate, in discussion with our expert genetic counsellors.
The information on this page is general in nature. All medical and surgical procedures have potential benefits and risks. Consult your healthcare professional for medical advice specific to you.