Screening and diagnostic tests

Screening tests in pregnancy

Your doctor or midwife will offer you screening tests to find out if your baby is more likely to have a health condition. Screening cannot tell you if your baby has the condition.

You can choose whether to have screening tests or not. You will still get the rest of your antenatal care if you decide not to have any screening tests. The tests can help you make choices about any further tests and care or treatment during your pregnancy or after your baby's born.  

If screening shows that your baby has a higher chance of having a health condition, you will be offered a diagnostic test. This can tell you whether they have it or not.

You can choose to have screening for:

• Down’s, Edwards’ and Patau’s syndromes
• Down’s syndrome only
• Edwards’ and Patau’s syndromes only.

Some of these tests may not be suitable if you are expecting more than 1 baby. Your doctor or midwife can tell you what your options are.  

Combined test  

This combines a blood test with an ultrasound scan to screen for 3 genetic conditions ‒ Down’s syndrome, Edwards’ syndrome and Patau’s syndrome. It is not harmful to you or your baby.

The combined test involves:

• a blood sample taken from your arm between 10 and 14 weeks of pregnancy
• a nuchal translucency scan between 11 and 14 weeks of pregnancy.

The nuchal translucency scan is a normal ultrasound scan but the sonographer looks carefully at how much fluid is at the back of your baby’s neck. All babies have some fluid but babies with Down’s, Edward’s or Patau’s syndromes often have more fluid.  

Your antenatal team use the results of your blood test and the scan, together with your age (because some conditions are more likely in pregnancies where the woman or birthing person is older), to work out the chance of your baby having one of the conditions being tested for. 

Sometimes it is not possible to measure the nuchal fluid during the scan, for example if your baby is lying in a difficult position. You may be offered another scan on the same day or another day, or you may be offered the quadruple test (see below).

Quadruple test

You may be offered the quadruple test if:

• it was not possible to measure the nuchal fluid during the nuchal translucency scan, or
• you are more than 14 weeks pregnant when you have your first ultrasound scan.

The quadruple test is a blood test that screens for Down’s syndrome. You can have the test between 14 and 20 weeks of pregnancy.

It is not as accurate as the combined test.

20-week scan

This is an ultrasound scan, which is offered between 18 and 21 weeks of pregnancy. It is sometimes called the mid-pregnancy or fetal anomaly scan.

The scan checks your baby’s physical development and looks for 11 rare conditions. Some conditions are easier to see on the scan than others.

NHS has more information about the 20-week screening scan.  

Screening test results

The screening tests cannot tell you if your baby has a medical condition. The results will show whether you have a higher or lower chance of having a baby with Down’s, Edwards’ or Patau’s syndromes.

If you have a lower chance result, it is unlikely that your baby has one of these conditions but it is still possible. You will not need any further screening tests. Most screening test results (around 97% of them) are lower chance. 

If you have a higher chance result, it is not certain that your baby has a condition, but it is more likely. Overall, around 3% of screening test results are higher chance. In this case you may choose to:

• not have any further tests
• have another screening test that is very accurate, called NIPT
• have a diagnostic test.

Getting a screening test result that shows you have a higher chance of a condition is likely to cause you and your family anxiety. Your antenatal team will give you support and information to help you decide what you would like to do. It is important to speak to them if there is anything you do not understand or if you are worried about the test results.

The charity Antenatal Results and Choices aims to give you information and specialised support. They will not direct your decision, but they can help you make sense of what you have been told and gather the information you might need. ARC can help you make a decision that feels best for you and your family. Their helpline is answered by trained staff.  

Call 020 7713 7486 (Monday-Friday, 10am-5.30pm) or email [email protected] to arrange an evening call (Tuesday and Thursday, 8pm-10pm).

Non-invasive prenatal screening test (NIPT)

NIPT is a blood test that can give a more accurate result than the combined or quadruple test. It is not harmful for you or your baby.

The NIPT looks at your baby’s genetic material that has passed into your blood. This can show whether your baby is more likely to have a genetic condition.

If the results show your chance is lower, your baby is not likely to have Down’s, Edwards’ or Patau’s syndromes.

If you have a higher-chance result, you will be offered a diagnostic test.

Sometimes, the NIPT test does not give a result. If this happens, you can choose whether to:  

• repeat the NIPT test
• have a diagnostic test
• not have any further testing.

Diagnostic tests during pregnancy

A diagnostic test can tell you whether your baby has a genetic condition.

You will be offered a diagnostic test if:  

• you had a higher-chance screening test result
• your 20-week scan showed anything unusual
• you have had a pregnancy or baby with a health condition before
• either of the baby’s biological parents have a family history of health conditions, such sickle cell disease, thalassaemia or cystic fibrosis.

You do not need to have a diagnostic test if you do not want one. Your doctor or midwife will talk to you about your options and help you reach a decision that is right for you and your family.  

There are 2 diagnostic tests available: chorionic villus sampling (CVS) and amniocentesis. Both options have a 1 in 200 (0.5%) chance of causing a miscarriage. The chance may be higher if you are expecting more than one baby – your midwife will talk to you about your options.

Chorionic villus sampling (CVS)

You will usually be offered a CVS between weeks 11 and 14, but it can be done later.

It is important to remember that you do not have to have CVS if it is offered. It is up to you to decide whether you want it.

A midwife or doctor will speak to you about what the test involves and let you know what the possible benefits and risks are to help you decide.

The doctor passes a thin needle through your tummy, to take a tiny sample from the placenta. You will have a local anaesthetic to numb your skin. The doctor will use ultrasound to guide the needle to the right place.

Occasionally CVS is done by passing a thin tube or small forceps through your cervix instead.

Having CVS can feel uncomfortable – some people say it feels like having period pains. You may get some cramping for a few hours afterwards. Paracetamol can help with this. Always stick to the recommended dose on the packet.

Amniocentesis

You may be offered amniocentesis between 15 and 20 weeks, but it can be done later.

It is important to remember that you do not have to have amniocentesis if it is offered. It is up to you to decide whether you want it.

A midwife or doctor will speak to you about what the test involves and let you know what the possible benefits and risks are to help you decide.

The doctor takes a small sample of amniotic fluid by passing a thin needle through your tummy. They will numb your skin with a local anaesthetic and guide the needle into place using ultrasound.

Occasionally, the sample may not be large enough for testing and the doctor will need to re-insert the needle. If they are still not able to take enough fluid, you will be invited to come back another day.

Most people say that amniocentesis feels uncomfortable rather than painful. It can feel a bit like period pains. You may get cramping for a few hours afterwards, which you can take paracetamol for. Always stick to the recommended dose on the packet.

More information

Antenatal Results and Choices has information about antenatal tests and offers support if you are worried about the results- and unsure what you want to do next.

NHS provides information about screening for Down’s syndrome, Edwards’ syndrome and Patau’s syndrome in England.

NHS Inform has information about screening for Down’s syndrome, Edwards’ syndrome and Patau’s syndrome in Scotland.

NHS Wales provides information about screening for Down's syndrome, Edwards' syndrome and Patau's syndrome in Wales.

NIDirect provides information about tests to find abnormalities or chromosomal conditions in Northern Ireland.