Screening for Down's syndrome, Edwards' syndrome and Patau's syndrome

At around 11 to 14 weeks of pregnancy, you have a dating scan to see how far along in your pregnancy you are and to check your baby's development. This is also called a nuchal scan, or first trimester scan. 

When you arrive at the fetal medicine unit for your scan, we will offer you a blood test that screens for Down's syndrome, Edwards' syndrome and Patau's syndrome.

Combined screening cannot say for certain whether your baby has one of these conditions. It tells you if your baby has a lower or higher chance of having Down's syndrome, Edwards' syndrome or Patau's syndrome.

The NHS website has more information and an animation about screening tests in pregnancy.

How we do the combined screening test

If you would like to have screening, we take a blood test before the scan in the antenatal clinic room. The receptionist will be able to guide you there.

We analyse your blood while you have your scan and the sonographer (the professional who does the scan) will ask you to wait in the fetal medicine reception until the blood results are ready.

The results are usually available after the scan but if the results take too long, we'll let you know if you need to go home. If you go home before you get the results, we post them to you within 24 hours. We will also phone you if your results are higher-chance.

To improve the quality of the ultrasound images your bladder should be full. If your baby is not in the best position for the sonographer to complete the scan, we might ask you to walk a little to encourage your baby to change position.

The scan and the blood test are safe and will not harm you or your baby.

How we calculate the chances

The chance of Down's syndrome, Edwards' syndrome or Patau's syndrome is based on a combination of:

  • your age (and some other factors such as your ethnicity, if you smoke or not, and if you have diabetes)
  • the measurement of nuchal translucency (NT) which is a small amount of fluid at the back of your baby’s neck
  • the size of your baby
  • the level of 2 hormones, called PAPP-A and free B-hCG, in your blood

Results of the combined screening test

The screening test will give you a higher-chance result or a lower-chance result of having a baby with Down's syndrome, Edwards' syndrome or Patau's syndrome.

Higher-chance result

A higher-chance result is when the screening test shows a result anywhere between 1 in 2, and 1 in 150. For example, 1 in 35 would be a higher-chance result.

A higher-chance result does not mean your baby definitely has Down's syndrome, Edwards' syndrome or Patau's syndrome.

Lower-chance result

If the results show a risk of 1 in 151 or more, this is classified as a lower-chance result. For example, 1 in 300 would be a lower chance result.

A lower-chance result does not mean there's no chance at all of the baby having Down's syndrome, Edwards' syndrome or Patau's syndrome.

If you have a higher-chance result

If your screening test has a higher-chance result for Down's syndrome, Edwards' syndrome or Patau's syndrome, we will tell you about tests that can give you a more definitive 'yes' or 'no' answer.

If you have difficulty deciding what to do or would like further counselling, the fetal medicine midwives or the screening coordinator will be able to discuss options with you.

Further screening

You can decide to have further screening through a test called non-invasive prenatal test (NIPT). We do this with an additional blood test. This screening test is more accurate than the combined screening test.

Diagnostic tests

If you prefer, we can also offer you a diagnostic test. Based on how many weeks pregnant you are, this will either be: 

Both procedures are invasive and are performed by a fetal medicine consultant.

These diagnostic tests can confirm whether or not your baby has Down's syndrome, Edwards' syndrome or Patau's syndrome. However, these tests carry a 1% risk of miscarriage.

Alternatives to the combined screening test

There are other screening tests available which can be done at different stages of your pregnancy, such as the quadruple screening test. This test will be offered to you if the scan shows that you are more that 14 weeks pregnant.

Quadruple test

We offer you the quadruple blood screening test if:

  • you are more than 14 weeks pregnant, or
  • the sonographer is unable to measure the nuchal translucency because of a difficult fetal position, fibroids, uterine scars or a high BMI

This test can be done between 14 weeks and 2 days, and 20 weeks of pregnancy. This only screens for Down's syndrome. The results for this test will be sent to you by post or the fetal medicine midwives will call you if you have a higher-chance result.

If you decide not to have the combined test (decline screening)

The screening for Down’s syndrome, Edwards’ syndrome and Patau’s syndromes is optional. You do not have to have it.

If you decide not to have the screening test, there is no need for you to have your blood taken at your scan appointment. However, we will still offer you an ultrasound scan to give you a due date and check your baby’s development.

More information and support

GOV.UK has information about all the screening tests you will be offered during your pregnancy and after your baby is born. It explains the different types of test and what the tests are looking for.

Screening tests for you and your baby (STFYAYB) - GOV.UK (www.gov.uk)

GOV.UK also provides this information in easy read.

Screening tests for you and your baby: easy guides - GOV.UK (www.gov.uk)

Antenatal Results and Choices gives impartial information and support to help you decide on your next steps.


Positive About Down Syndrome is a parent-led website with stories and photos demonstrating the reality of life with Down's syndrome. They also offer online peer support.


Downs Syndrome Association offers information, support and a helpline to expectant and new parents.


SOFT UK provides support and information to families, healthcare professionals and carers affected by Patau’s syndrome (Trisomy 13) or Edwards’ syndrome (Trisomy 18) and their related conditions.


Important information about your ultrasound scan

The sonographer doing your scan needs to concentrate to get accurate measurements of your baby. For this reason, if you bring your children with you, it's important to have another responsible adult that can look after them and they may need to wait outside the room if they are crying or unsettled.

Mobile phones must not be used during the scan. The sonographer will allow you to take a picture with your phone once the examination is completed.

Please note that if you are more than 15 minutes late, you will not be able to have your scan and we'll offer you the next available appointment.

Resource number: 2319/VER10
Last reviewed: July 2023
Next review: July 2026

Contact us

If your dating scan appointment is not convenient for you, please phone the fetal medicine reception to have your appointment rescheduled.

Phone: 020 7188 8003

Do you have any comments or concerns about your care?

Contact our Patient Advice and Liaison Service (PALS)

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