Antenatal tests: chromosomal anomalies and other conditions

Chromosomal anomalies and other conditions

Human cells usually have 46 chromosomes. Chromosomal anomalies happen when there are missing or extra chromosomes or the chromosomes are altered in some way.

There are many different types of chromosomal anomalies, many of which might cause physical and/or intellectual disability. Conditions caused by chromosomal anomalies include Down syndrome, Edwards syndrome and Patau syndrome.

There are other conditions that aren’t caused by chromosomal anomalies but that can develop in pregnancy. These include neural tube defects like spina bifida.

About antenatal tests for chromosomal anomalies and other conditions

Antenatal tests are the tests you have in pregnancy. These tests help you and your health professionals find out how likely you are to have a baby with chromosomal anomalies or other conditions.

There are two types of tests for chromosomal anomalies and other conditions – screening tests and diagnostic tests.

Antenatal screening tests work out the chance of your baby having certain chromosomal anomalies or other conditions. The results will say that your baby has a high chance or that your baby has a low chance of having a chromosomal anomaly or other condition. Screening tests don’t detect all chromosomal anomalies and don’t tell you whether your baby definitely has that condition.

If your screening test result says that there’s a high chance your baby has a condition, you’ll be offered diagnostic tests.

Diagnostic tests can give you a yes or no answer – yes, your baby has a condition, or no, your baby doesn’t have a condition. Diagnostic tests can also detect a wider range of chromosomal anomalies.

Your doctor or midwife will offer you screening and diagnostic tests, but these tests are optional. It’s your choice to have antenatal tests for chromosomal anomalies and other conditions.

Screening results use words like ‘high risk’ or ‘screen positive’ and ‘low risk’ or ‘screen negative’. The results might also show your risk as a number – for example, ‘1 in 500’. If you’re not sure what the result means for you and your baby, it’s OK to ask your doctor or midwife to explain.

Screening tests for chromosomal anomalies and other conditions

Early in your pregnancy, your doctor or midwife might talk with you about screening tests. These tests are safe for your baby and don’t increase the risk of miscarriage.

Non-invasive prenatal testing (NIPT)
Non-invasive prenatal testing looks at the chance of your baby having certain chromosomal anomalies.

NIPT involves a simple blood test. A small amount of your blood, which contains some DNA from your baby’s placenta, is tested.

You can have NIPT any time from 10 weeks of pregnancy onwards.

NIPT is more accurate than other screening tests, but it’s also the most expensive kind of screening test. You might also hear it called cell-free DNA or cfDNA testing.

Combined first trimester screening
Combined first trimester screening (CFTS) looks at the chance of your baby having certain chromosomal anomalies.

CFTS combines a blood test from you with a test from your 12-week ultrasound scan.

You can have the blood test at 8-12 weeks of pregnancy. It measures the levels of two different hormones that occur naturally in your blood.

The 12-week ultrasound is one of the common screening tests in pregnancy. If you decide to have combined first trimester screening, the professional doing the ultrasound will measure the fluid at the back of your baby’s neck. This is called the nuchal translucency.

Your baby might have a higher chance of chromosomal anomalies if:

you have certain levels of hormones
the amount of fluid at the back of your baby’s neck is higher than normal.

Second trimester maternal serum screening
Second trimester maternal serum screening looks at the chance of your baby having certain chromosomal anomalies.

This test involves a simple blood test. Your blood is tested for hormones from your placenta and from your baby. The levels of these hormones, your baby’s gestational age, and your age and weight are used to estimate the chance of your baby having certain chromosomal anomalies.

You can have this blood test at 14-20 weeks.

Second trimester maternal serum screening also looks at the chance of your baby having a neural tube defect like spina bifida. To confirm whether your baby has this condition, you’ll need to have an ultrasound, usually at 20 weeks of pregnancy.

Down syndrome and mother’s age
Screening tests for chromosomal anomalies will take your age into account. This is because the chance of having a baby with Down syndrome and certain other chromosomal conditions increases as you get older. For Down syndrome, at:

20-29 years, the chance is less than 1 in 1000
30 years, the chance is 1 in 890
35 years, the chance is 1 in 355
37 years, the chance is 1 in 220
40 years, the chance is 1 in 90
45 years, the chance is 1 in 28.

There are several things to think about before having antenatal tests. For example, how do you feel about what you might find out from these tests? What will you do if you get a high-risk result from a screening test? Would you go on to take a diagnostic test? It’s OK to ask your health professional for more information about the tests and what they mean if you’re not sure.

Next steps after screening tests

Your doctor or midwife will let you know the results of screening tests. If you get high-risk results, the doctor or midwife will talk with you about diagnostic tests.

If they don’t let you know about results, you have the right to ask.

Diagnostic tests for chromosomal anomalies

Depending on your health, your baby’s health and the results of your screening tests, you might be offered diagnostic tests to tell you more accurately whether your baby has a chromosomal anomaly.

Diagnostic tests involve taking small samples of tissue from your placenta or fluid from around your baby. There are two ways to take the samples – chorionic villus sampling (CVS) and amniocentesis.

Chorionic villus sampling (CVS)
CVS is used to get a sample of the placenta from your uterus.

First, a doctor gives you a local anaesthetic for your tummy area. Next, the doctor uses ultrasound to guide a thin needle into your uterus to collect one or more samples. If the position of your placenta makes it difficult or impossible to insert the needle through your tummy, the doctor will insert a soft tube through your vagina and cervix to get to your uterus – but this doesn’t happen often.

CVS is best done at 11-14 weeks of pregnancy – that is, in the first trimester. It might be up to two weeks until you get CVS results. Some clinics can return results faster (in 1-2 days), but this might cost more.

CVS has a risk of miscarriage of 1 in 100. This means that for every 100 times a CVS is done, there’s one miscarriage that would not otherwise have happened. It’s a good idea to discuss the risks with your health professional.

Amniocentesis
You might be offered amniocentesis as an alternative to CVS after 15 weeks of pregnancy. Sometimes, you might be offered amniocentesis if you’ve had CVS, but the CVS results aren’t clear.

Amniocentesis is used to take a sample of the fluid that surrounds your baby in the uterus. First, a doctor gives you a local anaesthetic for your tummy area. Next, the doctor uses ultrasound to guide a thin needle into your uterus to collect one or more samples.

Amniocentesis results might take a few weeks. Some clinics can return results faster (in 1-2 days), but this might cost more.

Amniocentesis has a risk of miscarriage of less than 1 in 200. This means that for every 200 times an amniocentesis is done, there’s about one miscarriage that would not otherwise have happened. It’s a good idea to discuss the risks with your health professional.

How your samples are tested
The samples taken by CVS or amniocentesis are sent to a laboratory for testing. The methods used to test the samples are usually karyotyping, fluorescence in situ hybridisation (FISH) or molecular karyotyping. Other types of tests can also be used to look for specific conditions.

For both CVS and amniocentesis, you need to go to a hospital or private clinic. It takes only a few minutes to get the sample, but you might be at the clinic for around two hours. You need someone to drive you home.

Costs of antenatal tests for chromosomal anomalies and other conditions

The costs of antenatal testing can vary a lot, depending on things like:

whether you go through the public or private health care system for your pregnancy
whether there’s a high chance that your baby has a chromosomal anomaly
who’s providing your pregnancy care.

It’s a good idea to check with your pregnancy health professional about costs, but here’s a rough guide:

Non-invasive prenatal testing (NIPT): you have to pay for this test. Depending on which test you choose to have, the cost can be $400-$600. You can’t get any money back from Medicare or private health insurance.
Combined first trimester screening: costs depend on your health care provider. You can get some money back from Medicare to cover part of the cost of the blood test and the scan.
Second trimester serum screening: you usually don’t have to pay for this test in the public or private health system.
CVS, amniocentesis and laboratory testing: you don’t have to pay for this test in the public health system. If you’re in the private system, you have to pay for this test but you can get some money back from Medicare.