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

As with all aspects of your pregnancy care, you will be asked whether you want to have this test or not. It is your individual decision but it is important to be fully informed so you can make the right decision for you.

What are these syndromes exactly?

All 3 syndromes are genetic conditions where a baby has an extra copy (or part of a copy) of a particular chromosome, leading to particular set of disabilities or growth problems. Down's syndrome is more common than both Edwards' and Patau's syndrome, which are very rare.

Edwards' syndrome or Trisomy 18 (T18) causes major learning disabilities, and can also cause heart problems, unusual head and facial features, and serious growth problems. Most babies with T18 will die before they are born, be stillborn or die shortly after birth.
Patau's syndrome or Trisomy 13 (T13) can also cause major learning disabilities and heart problems, as well as cleft lip and palate, growth problems, poorly formed eyes and ears, and kidney problems. Most babies with T13 will die before they are born, be stillborn or die shortly after birth.
Down's syndrome or Trisomy 21 (T21) can cause some growth delays, mild to moderate learning disabilities and distinctive facial characteristics. Many babies affected by it, if carried to term, grow up to live healthy and full lives – with, where it's needed, extra medical or specialist help.

How likely is it that my baby has one of these syndromes?

It's highly unlikely. It's thought that Down's syndrome affects 1 in every 1000 babies born, while the much rarer Edwards' syndrome affects 3 in every 10,000 pregnancies. Patau's syndrome affects approximately 1 in every 4000 pregnancies.

Will the combined screening test tell me for sure either way?

No, we're afraid it's a little bit more complicated than that. This test will only tell you whether there is a high chance or a low chance of you having a baby with Down's, Edwards' or Patau's.

If you're told your chance of having a baby with any of these conditions is low (as the vast majority of women are), you won't need any further testing; if you're told your change is high, you will probably be offered further, more accurate, tests. Some of these tests carry a (very small) risk of miscarriage.

"It's fair to say that 95% of women who have the combined screening test get results which are low chance," says Jane Fisher, director of the testing-support charity Antenatal Results and Choices (ARC).

Do I have to have the combined screening test?

No. It's completely up to you whether you have it or not. And, even if you do decide to have it, you don't have to be screened for all 3 syndromes, if you don't want to.

So, you can choose:

To have screening for Down's, Edwards' and Patau's
To have screening for Down's only
To have screening for Edwards' and Patau's only
Not to have the test at all

Many pregnant women decide they do want to have the complete combined screening test because, if it did come back as a high chance, they'd then want to have the other more accurate tests – and, if those came back positive, they could then make a decision either not to continue with the pregnancy or find out about their baby's future health needs and outlook.

For those who choose not to have the combined screening test, like Becci from our MadeForMums Community, it's often more of a gut decision that knowing more wouldn't make any difference.

"We didn't have any testing, over and above the normal ultrasound scan," she says. "We talked about it and we both agreed that we wouldn't terminate whatever, so the tests were redundant.

"But I don't think there's any right or wrong here. It's a very personal choice."

When do I have the combined screening test?

You'll be offered this test towards the start of your NHS antenatal care, when you're between 10 and 14 weeks pregnant.

What exactly does the testing involve? Why is it called 'combined'?

"It is called the 'combined screening test' because it combines the results of a blood test with a nuchal translucency (NT) measurement taken when you have your 12-week ultrasound scan," says Gill Harrison, professional officer for ultrasound at the Society and College of Radiographers,

This data is then further combined with your age and the gestational age (how many weeks old your baby appears to be, according how long he or she measures, head to bottom, during your scan).

You may have the scan and the blood test on the same day, or they may be done separately.

The blood test is the usual needle-in-the-arm job and may be done by your midwife or a phlebotomist (someone who specialises in taking blood sample). It's common for the blood sample needed for the combined screening test to be taken at the same time as blood is taken for the routine pregnancy blood tests (to check your blood type and so on), to save jabbing you in the arm on two different occasions.

"During the NT scan," says Gill, "the fluid in the space at the back of your baby's neck is measured by the sonographer who's conducting your ultrasound scan." (Babies with genetic conditions such as Down's tend to accumulate more fluid in this space than average at this stage of pregnancy.)

Occasionally, and usually because of the way your baby is lying, it's not possible for the sonographer to get an NT measurement. If this is the case for you, you should be offered a 2nd scan.

If it's still not possible to get a measurement then, you may then be referred for a quadruple test (a different blood test that screens for Down's but not Edwards' or Patau's) or, if your hospital offers it, a NIPT (a newer blood test that screens for dozens of conditions).

When will I get the results?

You may be told the NT test result on the day you have your scan. But the length of time it take to give you your final combined-test results varies and, obviously, often depends on whether you have the the scan and the blood tests done on the same day or not, and how long your hospital takes to process all the information gathered by the tests.

Generally speaking, you should hear in between 1 to 3 weeks.

What do the results mean?

If you're given the NT test result separately, you'll be told the fluid measurement in millimetres: a measurement of 3.5mm or more suggests your baby could have an increased chance of Down's, Edwards' and Patau's.

The results of the combined test are given as 2 scores: 1 for the chance of your baby having Down's and 1 for your chance of your baby having Edwards' or Patau's (combined).

"If you are told your chance is between 1 in 2 and 1 in 150, that's considered high," says Jane Fisher. But that doesn't mean your baby definitely has any of the 3 syndromes; it just means you may be offered other, more accurate tests.

Anything over 1 in 151 is considered low. “If your chance is reported as low," says Jane, "it is important to remember that this does not mean your baby definitely does not have one of these syndromes." It just means it is extremely unlikely – so unlikely, there's no need for further testing.

How accurate are the results? The combined screening test picks up more than 4 out of 5 (85 to 90%) babies with Down’s syndrome, Edwards' syndrome and Patau's syndrome.

But in 2 out of every 100 tests, the result may indicate that your baby has 1 of the syndromes, when he or she doesn’t: a 'false positive'.

If I'm told my chance is high, what happens next?

You will be offered an appointment with a midwife to discuss going on to have a further, more accurate tests. You don't have to have them if you don't want to.

You may be offered a new screening test called NIPT (also known as cffDNA), which is a blood test that offers a higher degree of accuracy in detecting Down's, Edwards' and Patau's.

If you don't have NIPT, or if the NIPT result is positive, you are likely to be offered either amniocentesis or CVS – both of which are called 'diagnostic' tests because the results are so accurate (99%) that, if it's positive for Down's, Edwards' or Patau's, doctors will view it as a diagnosis.

Both amniocentesis and CVS carry with them a small risk of miscarriage, so you'll have the difficult task of weighing up if it's worth taking that small risk to find out for (almost completely) sure.

Just be reassured that there will be lots of advice and support offered to you along the way.

Pics: Getty Images

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Rachel Mostyn