Genetic carrier screening

 Why have genetic screening before pregnancy?

Genetic carrier screening is recommended before pregnancy to assess the chance of having a child with a serious inherited condition.

Even if there is no known family history of genetic disease, this does not necessarily mean the risk is low. Many genetic conditions can be silently carried by egg or sperm provider without causing any health issues for them but still have the potential to affect a child.

Explaining gene variations

Our DNA contains the instructions that guide how the body develops and functions. This genetic information is organised into chromosomes, which contain smaller segments called genes.

Each person typically has 46 chromosomes, 23 inherited from the egg and 23 from the sperm. When these combine at conception, they create a unique genetic makeup.

During this process, genetic variations can occur. Some are inherited from the egg or sperm, while others may arise for the first time in that individual. Many variations are harmless, but some can interfere with how a gene works and may lead to a genetic condition. Carrier screening helps identify whether an individual carries such variations that could be passed on to a child.

What does it mean to be a carrier?

Being a carrier means you have a change in one of your genes that does not affect your own health but could be passed to your children. Most people carry several genetic variations without knowing it.

The more conditions included in a screening panel, the higher the chance of identifying a relevant variation. However, many genetic conditions are rare, and it is uncommon for both partners to carry the same condition. It is also not possible to test for every genetic disorder, and some conditions may occur spontaneously, affecting only that child.

Types of inherited conditions screened for

Genetic testing before pregnancy looks for autosomal recessive and X-linked recessive genetic conditions.

Carrier screening mainly focuses on two inheritance patterns:

Autosomal recessive conditions

If both egg and sperm carry the same autosomal recessive condition, there is a 25% (1 in 4) chance with each pregnancy that the child will be affected. The condition only occurs if the child inherits the altered gene from both parents. An example is cystic fibrosis.

X-linked conditions

These conditions are linked to genes on the X chromosome. The person providing the egg has two X chromosomes. If they carry a variation on one X chromosome, they are usually unaffected but can pass it on.

Male offspring who inherit the affected X chromosome may develop the condition.

Female offspring who inherit the affected X chromosome are usually carriers but typically unaffected.

An example is Haemophillia.

Because of this inheritance pattern, screening the person providing the egg is particularly important for identifying X-linked conditions.

Some genetic conditions follow different inheritance patterns, such as autosomal dominant disorders (e.g. Huntington’s disease). These are usually investigated separately if there is a known family history. It is important to inform your doctor if any genetic conditions run in your family.

If you are found to be a carrier

If a genetic variation is identified, your reproductive partner may also be offered testing to assess the combined risk.

If both egg and sperm providers are carriers of the same condition, a genetics specialist will discuss available options. These may include:

·         IVF with genetic testing of embryos (PGT-M)

·         Testing during pregnancy

·         Considering different reproductive options

·         Monitoring and planning care after birth

For further information about genetic carrier screening or how to access testing, speak with your doctor.