Screening tests will let you know whether your baby has a high chance of a particular condition, such as Down’s syndrome. Diagnostic tests will let you know whether they have it.
Screening tests in pregnancy
The results of screening tests will not tell you for sure whether your baby has a particular condition but they can show if there’s a higher risk. As with all tests in pregnancy, you can choose whether to have screening tests or not.
Different hospitals offer different tests – in some places certain tests are combined into one test. Ask your midwife how it works in your area.
Nuchal translucency scan
There is a little fold of skin at the back of your baby's neck called the nuchal fold. Everybody has one, but babies with Down's syndrome have been found to have a larger nuchal fold than other babies. The nuchal translucency scan helps your antenatal team work out how likely it is that your baby has Down's syndrome or some other conditions. This test is only valid when it is done between 11 and 14 weeks of pregnancy.
Combined test for Down’s syndrome
In many hospitals the results from the nuchal translucency scan are put together with a blood test that measures hormone levels from your placenta. When the two are looked at together, this is called the combined test. Your age is taken into account as well as the results of the scan and the blood test (the combined test) to give an indication of whether you have a high or a low chance of having a baby with Down’s syndrome.
The combined screening test picks up around 90% of babies with Down’s syndrome but it will also give a Down’s result to 5% of pregnancies where it is not true.
Triple or quadruple test
If you are more than 14 weeks pregnant when you have your first ultrasound scan, you will be offered a blood test called a triple (or quadruple) test. This is another way to assess your baby's chance of having Down's syndrome and can be done up to 20 weeks of pregnancy.
The quadruple screening test picks up around 80% of babies with Down’s syndrome but it will also give a Down’s result to 3% of pregnancies where it is not true
Screening tests only indicate chance
Remember that the nuchal translucency scan, combined test and triple or quadruple test give only an indication of the chance of your baby having Down's syndrome. None of these tests can tell you for certain. If the results show that your baby has a higher chance, you can have further tests that will diagnose the condition.
What does it mean if one of my screening tests is 'positive'?
Screening test results are sometimes talked about as being 'screen positive'. This simply means that you will be offered a diagnostic test to find out whether you or your baby have a particular condition.
Sometimes tests don't show a clear result and need to be repeated. Try not to worry - it doesn't necessarily indicate that anything is wrong.
If there's anything you don't understand, or you are worried about any tests or results, it's very important to ask your midwife to explain what the results mean for you and your pregnancy.
Diagnostic tests during pregnancy
If your screening tests do not show a higher chance of Down’s syndrome, you will not need a diagnostic test. Diagnostic tests are for women whose screening tests have shown a higher chance of problems and who would like to know more.
Chorionic villus sampling (CVS)
CVS is done between weeks 11 and 14. It can tell you if your baby has Down's syndrome or other conditions such as sickle cell anaemia and thalassaemia.
The doctor will take a sample from the placenta using a thin needle that is inserted through your tummy. This is done under local anaesthetic and using ultrasound as a guide to make sure the needle is in the right place. Occasionally CVS is done by passing a fine set of forceps through your cervix instead.
Both options have a 1 in 100 risk of causing a miscarriage.
Amniocentesis is done after 15 weeks and will diagnose Down's syndrome and other conditions in your baby.
In this test, the doctor will take a sample of the amniotic fluid that surrounds your baby. As with CVS, this is done with a thin needle inserted into your tummy and the doctor will use ultrasound as a guide.
Amniocentesis has a 1 in 100 risk of causing a miscarriage.
Non-invasive Prenatal Testing (NIPT)
There are new non-invasive tests that can check the chance of Down’s syndrome as well as other conditions. They are not available free on the NHS but can be done privately in some clinics at a cost of between £400 and £900. The test is done with an ultrasound scan and a blood test that looks at the baby’s DNA, a small amount of which is found in the mum’s blood. It can happen from 10 weeks up to the end of pregnancy. This test carries no risk to the baby, unlike CVS and amniocentesis, which both carry a 1 in 100 risk of causing a miscarriage.
5. Amniocentesis’, NHS Choices: http://www.nhs.uk/conditions/Amniocentesis/Pages/Introduction.aspx [accessed 10 February 2015] (last reviewed: 27 May 2014; next review due: 27 May 2016); RCOG (2010) Amniocentesis and Chorionic Villus Sampling: Green-top Guideline No. 8, London, Royal College of Obstetricians and Gynaecologists (reviewed: 2014).