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The nuchal translucency scan measures the amount of fluid at the base of your baby's neck. It can indicate a higher risk of genetic abnormalities, particularly Down's syndrome.
NICE guidance says all women should be offered the nuchal translucency scan and at some hospitals the test is given as part of your 12-week scan. If your hospital does not do this, you can ask for a referral or have the test done privately at varying cost.
The scan needs to be done between 11 and 13 weeks plus six days gestation. You do not have to have the test - it's your choice.
What happens during the scan?
During an ultrasound, the sonographer will measure the amount of fluid at the base of the baby's neck. On the scan it looks like a black space at the top of the spine. Any measurement of 3mm and above indicates an increased risk of Down's syndrome.
The sonographer will also check to see whether a nasal bone is visible on the scan. Studies have shown that three in four foetuses with Down's syndrome do not have a visible nasal bone at this stage of development.
The foetus will be measured and the precise gestational age calculated. This information, along with details of maternal age will be entered into a statistical analysis program and your specific odds of having a baby with genetic abnormalities will be calculated.
What does the scan show?
It is important to remember that after the nuchal scan you are given a percentage of risk, not a diagnosis, of Down's syndrome and other genetic abnormalities.
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What do the results mean and how reliable are they?
Before your scan, your chances of having a baby with Down's will be calculated based on your age only - the risks for women of various ages are:
20 years - 1 in 1,500
30 years - 1 in 900
40 years - 1 in 100
45 years - 1 in 30
The majority of women will receive a reassuring test result, as the risk will be lower than the risk calculated on age alone. In about 5% of cases the risk will be reassessed as higher after the nuchal scan test. Further investigations will be recommended for anyone with a risk higher than 1 in 150.
Combined screening - which involves examining the nuchal fold and doing a blood test that will look for the levels of pregnancy-associated plasma protein–A and free beta–hCG in the mother's blood - offers a slightly more informed risk percentage. The levels of the two hormones may indicate a higher risk of chromosomal abnormalities, although it is still not a diagnostic test and cannot be 100% accurate.
What are your options if you have a high risk of Down's syndrome?
As the nuchal test and the combined screening only give an indication of risk, you will be offered a more definitive test - chorionic villus sampling (CVS) or amniocentesis - that gives a more accurate indication of any genetic abnormalities.
These tests have much lower error rates, but are invasive and carry some risk, so it will be up to you to decide if this further testing is right for you.
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