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Innermost Secrets / Innermost Living / Body Clock
Skip Navigation LinksNuchal Scans
Page modified at: 28/08/2011
Traditional Screening for Down's Syndrome
Various tests have been developed to try to identify those women at greatest risk of having a chromosomally abnormal baby such as Down's Syndrome, who could then decide whether or not to proceed to amniocentesis. The TripleTest or Barts Test at about 15 weeks into the pregnancy detects about 70% of Down's Syndrome pregnancies based on about 1 in every 25 (4%) women having the blood test ending up with a high risk result (i.e more than 1 in 150) and then proceeding to amniocentesis. This means it misses 1 Downs Baby for every 2 it picks up!

Disadvantages of Serum Screening (Triple Test)
The test is less efficient in younger women with a detection rate of only about 30% and in women over 35 there is a high likelihood that testing will place them in a high risk category warranting amniocentesis. This means unecessary amniocentesis procedures and miscarriages which could have been avoided. This has been highlighted in a recent article in the Guardain.

NHS failure on Down's screening - click to read the article!

The blood test is not done until 15 weeks and therefore decisions about amniocentesis are made at 16 weeks with final results and decisions about the pregnancy made at 18 weeks. At this late stage the woman can feel the baby's movements and the pregnancy is almost half way through. If termination is considered for an abnormal result it is usually by a medically induced birth, which may mean up to 2 to 3 days in hospital.

The test is not valid for women with multiple pregnancy and cannot be relied upon in women who have had a previous baby with a chromosomal abnormality such as the common type of Down’s Syndrome (Trisomy 21).

If all is well then the woman will have had to wait until nearly 18 weeks into the pregnancy to have the reassurance that the baby does not have Down’s Syndrome if her blood test shows a high risk result.

Nuchal Translucency ScanNuchal Translucency Scan
A new test based on an ultrasound scan at about 12 weeks (11+3 to 13+6 weeks) has now been evaluated in studies of over 200,000 women and found to have a high detection rate across all ages. The centre is licensed and quality controlled by the Fetal Medicine Foundation in London who have pioneered this new approach to screening. All scans are performed by a Consultant in Fetal Medicine or a DMU certified sonographer or equivalent and all blood samples are processed by the Fetal Medicine Foundation in London.

By measuring the thickness of tissue fluid that lies under the skin at the back of the baby's neck, the Nuchal Translucency, a computer programme can convert this, the length of the baby and the mother's age into a risk for Down's Syndrome for that baby. Because it is based on the appearance of an individual baby, the technique can also be used in twins and triplets etc to give a risk for each baby. The test has a detection rate of 80% based on scan alone rising to nearly 90% in single pregnancies if blood markers (PAPP-A and free B-hCG) are also measured in a sample of blood taken from the pregnant woman’s arm at the time of the scan. Results based on scan are available on the day and blood tests results are available within a few days.

High Risk Results
If the risk is more than 1 in 150 at term, the NT scan definition of a "high risk" result, then further testing such as CVS or amniocentesis would be offered. This can either be as an NHS patient in the woman's own hopsital or privately through Innermost Secrets. Women should ask about their local unit's miscarriage rates as they vary widely but nationally are around 1%. In Innermost Secrets the rate is less than 0.5%.

Overall about 1 in 25 (4%) women having the scan would have to consider proceeding to CVS or amniocentesis.

Other Abnormalities
As a completely chance finding it has now been realised that a high Nuchal Translucency measurement also identifies babies at a higher risk of cardiac abnormalities. Cardiac abnormalities have only about a 40% detection rate in Wales based on the routine 18-20 week detailed anomaly ultrasound scan. Using a Nuchal Translucency gap of more than 3.5mm will identify about 1% of all pregnancies who could then be offered a proper detailed cardiac scan.  This would have a much better chance of identifying heart problems and planning the care of the pregnancy and delivery accordingly. These babies also have a higher chance of other rare syndromes and referral to a tertiary unit may be appropriate for further ultrasound assessment.

Other Advantages
The test is also valid for many women who have had a previous baby with chromosomal abnormalities like the common type of Down’s Syndrome (Trisomy 21) and is the only useful screening test for twins, triplets etc. It also identifies pregnancies at increased risk of other chromosomal abnormalities like Trisomy 18 (Edward Syndrome), Trisomy 13 (Patau Syndrome) and X0 (Turner Syndrome)

A New Approach to Screening
A woman having a Nuchal Translucency Plus scan at 12 weeks could have a Down’s risk assessment and the option of a rapid amniocentesis (now available at the Cardiff Fetal Medicine Centre) which gives diagnostic result for Down’s Syndrome in about 2 days if they have a high risk of Down’s Syndrome based on their scan. If so their final amniocentesis results would be available before the end of the 14th week when a surgical termination (D&C) can be safely done via the British Pregnancy Advisory Service although most NHS hospitals won’t do this after 13 weeks.

Women under 35 would have a higher chance of detecting Down's Syndrome and those over 35 would have a lower chance of a high risk result and therefore an amniocentesis by having a Nuchal Translucency Plus Scan rather than the Triple Test.

The vast majority of woman would however have had a highly reliable reassuring result before 14 weeks of pregnancy based on a scan and a blood test having avoided the difficult decision about amniocentesis and the risk of miscarriage from it.

Screening For Down Syndrome
 
Taking blood for the Triple Test
 
Nuchal Scan
(photographs courtesy of the Dr Eva Pajkrt, University of Amsterdam & Fetal Medicine Foundation, London)
 
Photograph of an Embryo at 12 weeks
(photographs courtesy of the Dr Eva Pajkrt, University of Amsterdam & Fetal Medicine Foundation, London)
 

Does maternal age affect the risk of a screen positive result or the detection rate?
Yes. A rough guide to the probability of having a screen positive result and the detection rate for Down Syndrome is shown below.

Maternal age (years) Probability of screen positive result Detection Rate (%)
Under 25 1 in 120 71
25-29 1 in 100 72
30-34 1 in 50 78
35-39 1 in 20 86
40-44 1 in 7 93
45 and over 1 in 4 95
All Ages 1 in 40 84
Mid-trimester test perfromance based on a 1 in 150 term cut-off

 

 

 
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