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Different Types of Antenatal Tests

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Screening tests

A screening test shows if you are likely to have a baby with particular disabilities. It cannot tell for certain whether your baby has a condition. Screening tests are usually either scans or blood tests and won’t affect your baby or harm his or her development in any way. These tests are often called ‘non-invasive tests’.

In most areas of the UK, all pregnant women will be offered screening for Down’s syndrome. This will be either a blood test or a nuchal translucency scan. If you choose to have the screening test and the results show your chance of having a baby with Down’s syndrome is more than 1 in 250 then you will be told you are ’screen positive’ or ‘high risk’. However, within this category there is a wide range. A risk of 1 in 100 is very different from a risk of 1 in 4, although both are described as being high risk. A 1 in 100 result means you have a 1% risk of your baby having Down’s syndrome and a 99% chance that he or she will not. But a risk of 1 in 4 means that your baby has a 25% chance of having Down’s. If your results are screen positive, or higher than 1 in 250, you will be offered another type of test (a diagnostic test).

Diagnostic tests

Diagnostic tests are used to find out for certain whether your baby actually has a specific condition or disability. Apart from the anomaly scan, diagnostic tests can affect your baby and have a slight risk of miscarriage. They are tests done on samples of fluid or tissue near the baby. These tests are often called ‘invasive’ tests.

The most common diagnostic tests are:

• Amniocentesis
• chorionic villus sampling (CVS)
• fetal blood sampling
• a detailed anomaly ultrasound scan

Amniocentesis - this is usually performed between 15 and 18 weeks of pregnancy. A tiny sample of amniotic fluid (the water surrounding the baby) is collected through a needle guided by a scan. The results usually take between two and three weeks. The cells from the sample have to grow so that the baby’s chromosomes can be checked.

Some hospitals now offer a special antenatal test to check for some conditions, and this can give the result in three days. This is a ‘molecular’ test and the cells do not need to grow to be checked for certain conditions. You would probably have to pay to have these special tests. The risk of miscarriage from having an amniocentesis is about 1 in 100 (0.5%-1%). After this test you should go home and rest for 24 hours.

Chorionic villus sampling (CVS) - this is usually performed between 11 and 13 weeks of pregnancy. A tiny tissue sample is taken from the placenta with a needle guided by a scan. Initial results are available after a week with full results ready in about three weeks. The risk of miscarriage from having CVS is about 2 in 100 (1-2%). After this test you should go home and rest for about 24 hours.

Fetal blood sampling - this test isn’t as common as the other tests. It can be carried out later in pregnancy at specialist centres. A sample of blood is taken from the umbilical cord to diagnose infections and problems with the blood. Results take between three and four days. The risk of miscarriage from fetal blood sampling is about 2 in 100 (between 1-2%).

Anomaly scan (also known as the 18 to 22 week scan) - This detailed scan checks the structure and organs of your baby, including the brain, heart and lungs. Some hospitals have better scanning equipment than others and the anomaly scan does not always pick up a specific problem. You can ask at your hospital about how well they detect problems. Although you do not have to wait for results, sometimes you may need to have the scan repeated. This might be because the picture is not clear due to the way the baby is lying or because the sonograpner (the person doing the scan) would like to refer you to someone else. There is no known risk of miscarriage from the anomaly scan.

Which is right for me?

You may not feel sure about which antenatal tests to ask for or accept. You may need to ask for more information before you decide whether a particular test is right for you. You can refuse any test or procedure that you do not want, or ask for more time if you are not ready to decide which test to have, although some antenatal tests need to be carried out at certain stages of pregnancy.

You may find it helpful to talk about the antenatal tests with your partner or family, other parents, your midwife or a counselor. Think about the advantages and disadvantages of having an antenatal test, and how you would feel if your baby has a disability. You should be given time to think about your options. But remember that most women have healthy babies.

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