Genetic and chromosomal screening and diagnostic testing in pregnancy
***Key Points***
Every baby has a small chance of being born with a chromosomal or genetic condition.
Testing during pregnancy for some of these conditions is called prenatal or antenatal screening.
These tests are offered to all women but it is your choice whether to have any prenatal screening tests.
You should only consent to testing if you understand what the test is for and what the results are able to tell you.
While most women will have a healthy baby, in approximately 1 in 25 pregnancies which is about 4% a baby will be diagnosed with a condition that may require specialised medical attention.
These conditions include physical and intellectual development and range from mild to severe.
There are two groups of tests you can opt for in pregnancy
- Screening tests
- Diagnostic tests.
***Key Points***
***Screening tests***
Screening tests: give an estimate or a risk of the chance that your baby is affected by a certain condition.
They involve taking a blood sample from your arm and an ultrasound and there is no risk of miscarriage.
Although many screening test are very sensitive they do not give you a definitive answer.
They tell you whether you have an increased chance of having a baby with a specific genetic condition.
If a screening test shows a high chance – then a diagnostic test is usually offered.
Examples of screening tests:
- Non-invasive prenatal test (NIPT)
- Combines First Trimester screening (CFTS)
- Second trimester maternal serum screening (2TMSS)
- Nuchal translucency ultrasound (NT scan)
***Screening tests***
***Diagnostic tests***
Diagnostic tests: are accurate tests that give a definite yes/no answer as to whether a pregnancy has a condition or not.
Diagnostic are commonly performed on samples of the placenta or amniotic fluid.
Examples
- Chorionic villus sampling (CVS)
- Amniocentesis
You can have a screening test and go onto have a diagnostic test if you wish.
***Diagnostic tests***
***Its a personal choice***
It a personal choice
Screening and diagnostic tests are voluntary and a personal choice.
Some women and families want to know in early pregnancy if their baby has a specific medial issue or chromosomal condition.
Some women would like to know as they would end the pregnancy.
Other women would like to know as they would like to be practically and emotionally prepared before their baby’s birth.
While some women opt not to have any testing as this would not impact their choices.
***Its a personal choice***
***What is screened for?***
What can be screened for?
The most common chromosomal cause of intellectual disability in children and adults is Down syndrome. This condition occurs when a baby has three copies of chromosome 21 (trisomy 21), instead of the usual two copies.
The chance if having a baby with down Syndrome increases with age. Down Syndrome occurs approximately 1 in 800 babies so this screening test is available to all women in Australia.
Other chromosomal conditions that may be screened for with the test are Edwards syndrome (trisomy 18) and Patau syndrome (trisomy 13). These conditions can have serious impact and increase the likelihood of pregnancy loss, or disability of death on a newborn baby with this genetic condition.
***What is screened for?***
***The NIPT Test***
Non-invasive prenatal test (NIPT)
- Is a screening test
- Offered between 10-16 weeks of pregnancy
During pregnancy, your baby’s placenta releases DNA into your blood stream.
The NIPT test is a simple blood test that analyses the DNA in your blood sample to detect chromosome conditions in the baby.
NIPT is the most accurate test for Down syndrome, and it can also test for many other chromosomal conditions. It is available from 10 weeks onwards.
The test is offered at private centres in Australia, not currently offered through Medicare and the cost varies where the test is performed but currently involves an out of pocket cost of between $350-$500
***The NIPT Test***
***Combined First trimester screening***
Combined First Trimester screening (CFTS)
Is a screening test
Offered between 9-13 weeks
This screening test involves an ultrasound between 11 – 13 weeks of pregnancy and a blood test between 10 -13 weeks of pregnancy. The ultrasound takes a measurement of the back of the baby’s neck (nuchal translucency) and is combined with the results of the blood test and your age to estimate the chance or risk of the baby having Down syndrome.
The NIPT test is also screening for Edwards syndrome (Trisomy 18, patau Sndrome (Trisomy 13) and turner Syndrome.
This test will predict approximately 85–90% of babies with Down syndrome, and may detect an increased risk of a range of other less-common chromosomal conditions. The ultrasound can also detect major structural conditions in your baby. There is some Medicare funding for this test, but usually involve some out-of-pocket costs to you ($100-$280).
***Combined First trimester screening***
***2nd trimester maternal serum screening***
Second trimester maternal serum screening (2TMSS)
- Is a screening test
- Offered between 14 – 20 weeks, best done between 15-17 weeks
This is a test offered to women in the second trimester of pregnancy to estimate the risk of their baby having Down syndrome.
This involves a blood test performed between 15 and 20 weeks of pregnancy.
It can detect approximately 75% of pregnancies with Down syndrome. It will also provide a risk of having a baby with Edwards syndrome and neural tube defects such as spina bifida.
Second trimester serum screening costs less than other tests, but is also less accurate than other screening tests
***2nd trimester maternal serum screening***
***Nuchal translucency ultrasound***
Nuchal translucency ultrasound (NT scan)
- Is a screening test
- Offered 11 – 13 weeks
The NT scan is an ultrasound scan that can also confirm your due date, identify if you’re carrying twins or confirm if a miscarriage has occurred.
The NT scan is often combined with a blood test as part of combined first trimester screening. It is also recommended to have this scan if you’re having non-invasive prenatal testing. The cost of a NT scan will vary depending on the provider
***Nuchal translucency ultrasound***
***Chorionic villus sampling***
Chorionic villus sampling (CVS)
- Is a diagnostic test
- Offered at 11 – 13 weeks
Chorionic Villus Sampling (CVS) is a medical procedure that samples a small amount of the placenta to confirm if your baby has a genetic disorder or chromosome condition.. It is usually performed at 11-13 weeks of pregnancy. CVS is usually done with a needle inserted through the woman’s abdomen (‘transabdominal’ CVS). Less commonly, a CVS can be performed through the vagina and the cervix (‘transcervical’ CVS).
A CVS is a procedure performed by a specialist doctor using ultrasound assistance. For a transabdominal CVS, the doctor cleans the skin on your abdomen with antiseptic. Local anaesthetic is injected to numb the skin before the CVS needle is inserted.
The ultrasound is used to guide the CVS needle safely through the skin into the placenta, avoiding any contact with the baby. The needle is only inside the womb for a minute or two. The pain from a CVS needle is similar to having a blood test from your arm, but there may be cramping, period-like pain during the procedure as well. For the vast majority of women, local anaesthetic is all that is required for the procedure.
There is a small risk of miscarriage during any pregnancy and having a CVS may increase the overall risk. Having a CVS may slightly increase your risk of miscarriage by 1 in 300 or less (< 0.3%). Your doctor should fully explain the CVS to you and obtain your written consent before performing the procedure.
You may consider a CVS if
- your screening test showed a high-risk result
- you have already had a child with a genetic or chromosomal condition
- you are over 37 years of age
- You or your partner are carriers of a particular condition and you would like to find out if this affects your baby
- you would like greater certainty about a diagnosis of Down syndrome or some other genetic condition
***Chorionic villus sampling***
***Amniocentesis***
Amniocentesis (amnio)
- Is a Diagnostic test
- Is offered at 15 – 21 weeks
An amniocentesis is a medical procedure to sample the fluid around the baby in the uterus (womb). It can be performed after 15 weeks of pregnancy and is a diagnostic test. An amniocentesis will confirm if your baby has a genetic or chromosome condition. The fluid is collected by a specialist doctor using a needle inserted through your abdomen. Ultrasound is used to guide the amniocentesis needle safely through the skin into a pocket of fluid inside the uterus, avoiding any needle contact with the baby. The procedure only takes a few minutes.
A small amount of fluid (about 1-2 teaspoons) is collected through the needle into a syringe and then transferred into a tube for laboratory testing. The needle only stays inside the uterus for a minute or two. There is no anaesthetic required, although you will be given a local anaesthetic to numb the skin and you will remain awake during the whole procedure.
There is a small risk of miscarriage during any pregnancy and having an amniocentesis may increase the overall risk. Having an amniocentesis may slightly increase your risk of miscarriage by 1 in 300.
Your doctor should fully explain the amniocentesis to you and obtain your written consent before performing the procedure.
You may experience some mild discomfort after the procedure. If you are a negative blood group, an anti-D injection will be given with consent after the procedure.
You may consider an amniocentesis if you
- your screening test showed a high-risk result
- you have already had a child with a genetic or chromosomal condition
- you are over 37 years of age
- You or your partner are carriers of a particular condition and you would like to find out if this affects your baby
- you would like greater certainty about a diagnosis of Down syndrome or some other genetic condition
***Amniocentesis***
***Ask yourself***
- If I choose not to proceed with screening am I comfortable knowing there is a small chance my baby may have a genetic or chromosomal condition?
- Is there any information about my baby that may mean I would end my pregnancy?
- Do I know what is being screened for in pregnancy?
- Do I understand what high and low risk means?
- If a screening test comes back as high risk, will I proceed with a diagnostic test?
- If a diagnostic test confirms a diagnosis, what would I do?
***Ask yourself***
Resources
https://www.mcri.edu.au/research/projects/prenatal-screening-diagnosis-fetal-chromosome-conditions
https://ranzcog.edu.au/wp-content/uploads/2022/05/Amniocentesis.pdf
