Nuchal scan

The nuchal translucency scan (also know as the nuchal fold test) is a measurement made during an ultrasound scan of 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. 

How to get a nuchal scan

NICE guidance says all women should be offered the nuchal translucency scan and at some trusts it is given routinely. However, not all hospitals offer the test. You can ask your doctor for a referral or you can have the test done privately at varying cost.

The scan needs to be done between 11 weeks and 13 weeks plus six days gestation. Some hospitals will offer the test as part of your 12-week scan. 

What happens during a nuchal scan?

During the scan the sonographer will measure the amount of fluid at the base of the 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 progam and your specific odds of having a baby with genetic abnormalities will be calculated. 

What do the results mean and how reliable are they?

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 risk of Down's syndrome and other genetic abnormalities.

Before your scan, your risk of Down's will be calculated based on your age only. According to the BBC website, the risks based on maternal age are:

  • 20 years - 1 in 1,500
  • 25 years - 1 in 1,300
  • 30 years - 1 in 900
  • 35 years - 1 in 350
  • 40 years - 1 in 100
  • 45 years - 1 in 30

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).

Screening tests cannot be 100% accurate and, according to NHS Choices, the nuchal translucency ultrasound 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 be high risk at all.

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. 

Combined screening

Combined screening involves examining the nuchal fold and doing a blood test that will look for the levels of two hormones: pregnancy-associated plasma protein–A and free beta–hCG in the mother's blood.

The levels may indicate a higher risk of chromosomal abnormalities. This 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. 

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 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.

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%. 

What Mumsnetters say about the nuchal scan

  • We were given a one in three risk of Downs and one in 36 of Edwards following the nuchal screening, mainly due to the blood test. I'm 35. Because the risks were so high we got lots of info and were fasttracked to invasive testing. There were no major chromosome abnormalities detected. The wait was the worst time of our lives though. Mandy73
  • At the end of the day, any screening test is just that - a risk assessment and not a diagnostic tool. The screening process is a tricky one as some will argue that if you won't proceed to diagnostic screening in the event of a high risk result (cvs or amino) then why screen in the first place? laluna
  • It's fine not to have nuchal screening, lots of people don't. Enjoy your pregnancy instead of stressing about false positives. brettgirl2
  • There are no such things as guarantees in life and disability can strike anyone at any time but we are very pro-screening and grateful for the choice. Mandy73
  • It's worth remembering that out of 4,000 people given a 1:4000 risk, one of them will have a baby with a chromosomal disorder, so there are no guarantees, even with excellent results. WhatFreshHellIsThis
  • The sonographers are used to emotional and scared ladies and I understand the nuchal scan is carried out by the most experienced. underactivethyroidmum
  • I'm quickly coming around to the fact that there seems to be LOTS of worrisome waiting during pregnancy. I never knew it could be so stressful. Schnitzel

 

Last updated: 03-Apr-2014 at 12:25 PM