Summary

Reproductive carrier screening is a way for people who want to have a baby to get tested for certain genetic conditions that can be passed down to their child. It may be performed before becoming pregnant (preferred) or in early pregnancy.

Why get tested?

Chromosomes are found in the nucleus of most of the cells in our bodies. They are the packaging for our DNA.  Humans have 23 pairs of chromosomes – one set comes from the father and one from the mother. This consists of 22 pairs of chromosomes called autosomes and one pair of chromosomes that determine gender called sex chromosomes.  

Typically, males have one X and one Y sex chromosome while females have two X sex chromosomes. A man can contribute an X or Y sex chromosome to his children while a woman can only contribute an X chromosome.  This means that the Y chromosome can only be passed down from father to son.

What are recessive conditions?
Many genetic disorders need both parents to carry mutations in the same gene for a child to be at risk of developing the condition. This is called a recessive condition.  

What are carriers?
Genetic conditions are caused by changes in genes that stop the gene working properly. Because we have pairs of chromosomes, one from each parent, we often won’t know if we have a faulty gene on one of the pairs, because the other, healthy gene can do the job. This is when we are said to be a carrier of the gene change.  If two people carry changes in the same gene, they have a one in four chance with each pregnancy to have a child affected by that condition.

What are X-linked conditions?
Things are a little bit different for X-linked conditions.  The condition will usually only appear in boys. Females have two X chromosomes so any faulty gene may be carried without her knowing – she can use the other one of the pair.  However, any boys she has must inherit one of her X chromosomes and he has no ‘spare’ X – dad gave them his Y chromosome instead. This means the mother has a one in four chance of having an affected boy.

Why carrier screening?
Because most carriers of recessive conditions have no family history of the condition, carrier screening should be considered regardless of whether or not there is a family history.

Although many conditions can be screened for, the most common testing looks at three genetic conditions:

• Cystic fibrosis (CF) is the most common life-limiting genetic condition in Australia.  About 1 in 30 people carry disease causing mutations in the CF gene.
• Spinal Muscular Atrophy (SMA) is the most common genetic cause of death in children under the age of two. About 1 in 50 people carry this condition.
• Fragile X syndrome (FXS) is the most common form of inherited intellectual disability. About 1in 6,000 females are carriers.

The test usually looks at the DNA from a blood sample from the female partner first.  If she is found to carry CF or SMA, a sample can be collected from the male partner to perform carrier testing for the same condition.  If both parents are carriers, there is a one in four chance of having a child affected by the condition.

If the woman is found to be a carrier of FXS, testing of the male partner is not required as this condition is carried on the X-chromosome.  If a woman is a carrier of FXS, she has a one in four chance of having a son with that condition in each pregnancy.

Carrier screening gene panels

Some laboratories offer screening for a wider range of conditions.  Because many more genes are screened there is a very high chance of one partner being a carrier for one or more of these conditions.  For this reason, this type of testing is usually performed on both partners.  Only those small numbers of couples where both are carriers for the same condition is there an increased risk of having an affected child.

Having the test

Sample

Blood

Preparation

None
 

Your result

Three-gene screening (CF, SMA, Fragile X)

If a genetic variant is detected and you are found to be a carrier of one or of these conditions, there could be an increased risk of having an affected child. The next step is usually to test your partner.
If you both carry variants in the same gene you are at increased risk of having an affected child, your doctor may refer you to a genetic counsellor.  Genetic counsellors are trained to interpret complex test results, help people understand their level of risk and what their reproductive choices are.

Expanded Carrier screening panels (many genes)

Carrier screening panels test for a wider range of severe, childhood-onset genetic conditions. They also include X-linked conditions that are less common.

People from some specific ethnic groups are more likely to carry specific genetic mutations than others. In this case, a doctor may recommend several other screenings.

An expanded carrier screen looks for many genetic variants including many rare conditions. Because there is a high chance of detecting gene alterations for one or more condition this is usually performed on couples at the same time and often includes consultation with a genetic counsellor prior to deciding to be tested.  Around 1 in 20 couples will both be carriers of the same condition and will be referred to a genetic health professional.  A negative test does not mean there is no chance of having a baby with a rare disorder but having a negative result can show that the likelihood is greatly reduced.