Nuchal translucency scan
The nuchal scan (NT scan) is a screening done during an antenatal ultrasound to measure the amount of fluid at the base of your baby's neck. This measurement can indicate a higher risk of genetic abnormalities, particularly Down's syndrome.
Getting a nuchal scan
All pregnant women will be routinely offered the nuchal translucency scan as part of their antenatal care. The scan needs to be done between 10 weeks and 13 weeks plus six days gestation, so many hospitals will include the test as part of your 12-week scan. It is up to you whether you want to have it.
What happens during a nuchal scan and what do they measure?
During the scan, the sonographer will measure the amount of fluid at the base of your baby's neck. On the scan it looks like a black space at the top of the spine. All babies have fluid at the base of the neck, but many babies with Down's syndrome will have an increased amount of fluid. A measurement of 3mm and above indicates a higher risk of Down's syndrome.
At the same time, the sonographer will 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 the mother's age will be entered into a statistical analysis program and the odds of your baby being born with genetic abnormalities will be calculated.
The scan cannot tell you conclusively whether or not your baby will have Down's syndrome – it can only estimate the level of risk. For example, if your result is one in 5,000, that means that if you had 5,000 babies, one would have Down's. If your result is one in 150 or less, it is considered high.
What do the results mean? At the end of the day, any screening test is just that – a risk assessment. It's not a diagnostic tool.
It is important to remember that after the nuchal scan you are given a percentage of risk, not a diagnosis. It is an indication of the likelihood of Down's syndrome and other genetic abnormalities.
Before your scan, your risk will be calculated based on your age only. These are the maternal age risks according to the NHS:
- 20 years of age has a 1 in 1,500 chance
- 30 years of age has a 1 in 800 chance
- 35 years of age has 1 in 270 chance
- 40 years of age has a 1 in 100 chance
- 45 years of age has a 1 in 50 or greater chance
In about 5% of cases the risk will be reassessed as higher after the nuchal scan test. The majority of women will receive a reassuring test result, as the risk will be lower than the risk calculated on age alone.
Further investigations will be recommended for anyone with a risk higher than 1 in 150 (although some NHS trusts use 1:300 as the cut-off point).
Most hospitals offer a combined test, which includes a blood test alongside the nuchal scan. This test will look at the levels of two hormones: pregnancy-associated plasma protein–A and free beta–hCG in the mother's blood. These levels may indicate a higher risk of chromosomal abnormalities.
This information will be combined with the data from the nuchal measurements along with the general risks for the mother based on her age.
Combined screening offers a slightly more informed risk percentage, although it is still not a diagnostic test and cannot be completely accurate.
How accurate is the nuchal translucency test?
Screening tests cannot be 100% accurate and, according to NHS Choices, the nuchal translucency scan has a false-positive rate of between 3.5% and 5%, which means that about 1 in 20 foetuses said to be in the high-risk category will not have genetic abnormalities.
It has a false-negative rate of 30%, which means that 70 of every 100 cases of Down's will be detected by the test.
What are your options if you have a high risk of Down's syndrome?
We were given a one in three risk of Down's following the combined screening, mainly due to the blood test. As the risks were so high we got lots of info and were fast-tracked to invasive testing. There were no major chromosome abnormalities detected, but the wait was the most difficult bit.
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 – to give a more accurate indication of any genetic abnormalities.
CVS and amniocentesis have much lower error rates: amniocentesis has a false-positive rate of 0.1% to 0.6% and a false-negative rate of 0.6%; CVS has a false-positive rate of 1% to 2% and a false-negative rate of 2%.
These tests are invasive and carry the risk of miscarriage (about 1 in 100, according to the NHS) so it will be up to you to decide if this further testing is right for you.
What Mumsnetters say about nuchal scans
“It's such a worrying time. The best thing you can do is try to forget about the tests until you get the results, as the discussing, backwards and forwards, and the waiting was what drove me crazy.”
“There's no such things as guarantees in life and disability can strike anyone at any time, but I was very pro-screening and was grateful for the choice either way.”