This is how genetic carrier screening works and what it tests for

Pregnancy can be an uncertain time and it's very normal to worry about the health of your baby.

While routine pregnancy screenings will pick up on any health concerns, you don't actually have to wait until you're pregnant to find out whether your baby is at risk for certain conditions.

Genetic carrier screening can determine the likelihood of your baby inheriting a genetic condition before it's even conceived and ultimately, help you feel more informed about your reproductive decisions.

What are carriers in genetics?

We all have about 23,000 pairs of genes and they all come from our parents. Genes determine everything from your physical characteristics to your blood type and even some of your health conditions.

It's why might get your blue eyes from your dad, your height from your mum and your curly hair from your grandmother. While that might sound simple enough, the structure of a gene is a little more complicated.

Every gene pair is made up of two copies of the same gene — one from each parent. A genetic carrier is someone who carries an abnormal copy or variant of a gene that happens to be associated with a health condition or disease.

The carrier usually isn't affected by the disease themselves but can unknowingly pass on this gene to their child. This won't matter unless the genetic carrier's reproductive partner also carries this variant and the couple's child inherits two abnormal copies of the gene.

The combination of these variants is what causes the inherited genetic condition to arise.

The two types of inheritance patterns

There are two main ways that genetic conditions are passed on and these are autosomal recessive inheritance and X-linked inheritance. In autosomal recessive inheritance, a child inherits a genetic condition because their parents are both carriers of the same genetic variant.

X-linked inheritance is more related to the sex of the individual and occurs when there is a faulty gene on the X chromosome. Since males only have one X chromosome, they are usually more affected than females.

What is genetic carrier screening?

Reproductive genetic carrier screening is a non-invasive prenatal test that identifies the risk of your child being born with a genetic condition. It does this by testing your and your reproductive partner's genes for the variants that are known to cause certain genetic conditions.

Genetic carrier screening should not be confused with routine pregnancy screenings or newborn screenings. These screenings are a way of testing your baby's genetics while genetic carrier screening is a way to test your own.

The other main difference is that genetic carrier screening is usually done before you become pregnant so that you feel more informed about future reproductive decisions.

The non-invasive prenatal testing is usually done via a saliva or blood test. The saliva or blood sample is then examined for the variants that are known to cause genetic health conditions.

Genetic carrier screening options include sequential or couple screening. If you choose to have sequential screening then only one member of the reproductive couple is screened.

In this case, the second member of the couple is only screened if the first member is found to be a genetic carrier of a condition. Alternatively, a couple screening tests both members of the couple at the same time.

What does genetic carrier screening test for?

There are hundreds of different inherited genetic conditions that can affect our health but most of them are very rare. Genetic carrier screening tests for some of the most common inherited genetic disorders including thalassaemia, cystic fibrosis, spinal muscular atrophy and fragile X syndrome.

The specific genetic conditions that the screening test looks for depend on the type of genetic screening test that you have.

The different types of genetic carrier screening tests include a 3-gene carrier screen, an expanded carrier screen and an ethnic-specific carrier screen.

• 3-gene carrier screening: This tests for the three most common genetic conditions, including cystic fibrosis, spinal muscular atrophy and fragile X syndrome.

• Expanded carrier screening: This one tests for the three most common genetic conditions along with hundreds of other rarer genetic conditions.

• Ethnic-specific carrier screening: This tests for genetic conditions that are known to be more common to certain ethnic groups. For example, couples of Eastern European (Ashkenazi) Jewish descent might be tested for Tay Sachs disease, Niemann Pick disease type A, Fanconi anaemia group C, familial dysautonomia, Bloom syndrome and Canavan disease.

Who should have genetic carrier screening?

While genetic carrier testing is particularly important for people who have a known family history of a genetic condition and people of some ethnic descents, anyone who is planning on having a baby in the future should consider taking a genetic carrier screening test.

While inherited genetic conditions are rare, the statistics show that a surprisingly high proportion of the general Australian population are actually genetic carriers of a serious or life-threatening genetic condition.

According to a study by the Genetics Society of America, most people carry an average of one to two genetic variants that can cause severe genetic disorders or prenatal death when two copies of the same variant are inherited [1].

In Australia, researchers have discovered that almost 1 in 20 people are genetic carriers of at least one of the three most common conditions [2]. If you and your partner happen to be a genetic carrier of the same condition, your child has a one in four chance of being affected by that condition.

Since the majority of genetic carriers aren't affected by the genetic condition themselves or even have a family history of the condition, they can end up unknowingly passing this condition onto their child.

This is why the Royal Australian and New Zealand College of Obstetricians and Gynaecologists recommend genetic carrier screen for all couples who are considering having children together [3].

Is genetic carrier screening right for you?

While genetic carrier screening is recommended for most people who are planning on having children in the future, it is still an optional test and a very personal decision that's only yours to make.

You might decide to take a genetic carrier screening test because you have a known family history of a genetic condition or you might simply want to feel more informed about your genetic risks.

For many women, being informed of their genetic risks leads to them feeling more empowered to make reproductive decisions that are right for them and their family.

Being informed about your reproductive risks can also reduce the risk of being caught off guard during your pregnancy and help you feel more prepared for any issues that may arise when it comes to your baby's health.

Genetic carrier screening is right for anyone planning on having children and wanting to feel more informed about their reproductive risks.

Can I have a genetic carrier screening test if I'm already pregnant?

Yes, if you're already pregnant then you can still have a reproductive genetic carrier screening performed.

It's usually better to have a reproductive carrier screening before pregnancy so that you can avoid potentially passing on a genetic condition but if you're already pregnant, a screening can still inform your future reproductive decisions.

Where to undergo genetic carrier screening

To arrange a genetic carrier screening, you can speak to your doctor or obstetrician and they'll refer you to a pathology collection centre.

What happens after carrier screening?

If you and your reproductive partner are found to be genetic carriers of a condition then you will be given information about your reproductive options and access to genetic counselling services.

Your reproductive options include:

• A natural pregnancy where the baby is tested for the condition once they are born
• A natural pregnancy where the foetus undergoes diagnostic testing in the womb
• Conceiving via in vitro fertilisation (IVF) and testing the embryos through preimplantation genetic diagnosis with only unaffected embryos selected for pregnancy
• Using sperm, egg or embryo from unaffected donors
• Adopting a child
• Not having children

Your genetic counsellor will give you more information about each option and help you make the right decision for your family.

You may also be given access to community resources, patient support groups and specialists who can give you more information on raising a child with the condition that you carry.

This will give you the opportunity to feel more informed, empowered and prepared for the reproductive path that you choose to take.