What is nuchal translucency?
Nuchal translucency is a collection of fluid under the skin at the back of your baby’s neck. The amount of fluid is measured during a nuchal translucency (NT) ultrasound scan:
- between 11 weeks and 14 weeks of pregnancy
- or when your baby measures between 45mm (1.8in) and 84mm (3.3in)
All developing babies have some fluid at the back of their neck. But many babies with Down’s syndrome have an increased amount.
That’s why the NT scan is used to help screen for Down’s syndrome.
Why might I have a nuchal scan?
All pregnant women are offered an NT scan, to help assess whether or not their babies are likely to have Down’s syndrome.
Every woman has a chance of giving birth to a baby with Down’s syndrome. The chance increases as you get older.
A screening test tries to get a clearer estimation of whether your baby may have Down’s syndrome. It can’t tell you for sure.
For example, if the result of your scan shows that you have a one in 1,000 chance, this means that for every 1,000 babies with your level of likelihood, one will have Down’s syndrome. A chance of one in 150 or less is considered high.
Diagnostic tests, such as chorionic villus sampling (CVS) and amniocentesis, can say for sure whether or not your baby has Down’s syndrome.
However, diagnostic tests carry a small risk of miscarriage. Because of this, your obstetrician will want you to have a screening test before she’ll offer you a diagnostic test.
The screening test combines the NT scan result, with a blood test and other factors, such as your age. The test is designed to give you as much information as possible, so you can decide whether or not to go ahead with a diagnostic test.
How is the nuchal scan performed?
An NT scan must take place at a particular time in your pregnancy. This is between 11 weeks and 13 weeks plus six days, to be exact, or when your baby’s crown rump length (CRL) is between 45mm (1.8in) and 84mm (3.3in). So the NT scan will usually happen alongside your routine dating scan.
It’s difficult to do the scan before 11 weeks because your baby is still so small. It would also be too early to combine it with your first trimester blood test.
It’s too late to do the NT scan after 14 weeks, as any excess nuchal fluid may be absorbed by your baby’s developing lymphatic system.
The scan is usually done through your tummy using normal ultrasound equipment. The person carrying out the scan (sonographer) will put some gel on your tummy and move a hand-held device (transducer) over your skin. It shouldn’t hurt, though you may feel some pressure as the sonographer presses the transducer into your skin to get a good view.
Sometimes, it’s hard to get a good view through your tummy, in which case the sonographer will ask your permission to carry out a vaginal scan. This is particularly likely if your womb (uterus) tilts backwards, or if you’re overweight. It’s safe for you and your baby, and shouldn’t be too uncomfortable either.
To date your pregnancy accurately, the sonographer will measure your baby from the top of his head to the bottom of his spine. She’ll then measure the width of the nuchal fluid at the back of your baby’s neck. The skin will appear as a white line, and the fluid under the skin will look black.
You’ll usually be able to see your baby’s head, spine, limbs, hands and feet on the screen. Your sonographer will be able to rule out some major abnormalities, such as problems with your baby’s abdominal wall, stomach, and skull, at this scan.
You’ll also be offered a more detailed anomaly scan between about 18 weeks and 21 weeks of pregnancy. By this time, the sonographer can see your baby’s features better, so the anomaly scan gives a better idea of how your baby is developing.
It may help if your partner or a friend goes with you to the scans, to share the experience and be there when you get the results.
How accurate is the nuchal scan?
The scan on its own picks up about 77 per cent of babies with Down’s syndrome.
Sometimes, a scan can suggest that a baby has a high chance of having Down’s syndrome, even though the likelihood is actually low. This is called a false positive.
The false-positive rate for an NT scan is five per cent. This means that one woman in 20 is wrongly given a high-chance calculation.
Combining an NT scan with a blood test gives a more accurate result. The blood test measures the levels of the hormone, free beta-hCG, and the protein, PAPP-A. Babies with Down’s syndrome tend to have high levels of hCG and low levels of PAPP-A.
When the NT scan is combined with this blood test, the detection rate increases to 90 per cent. This is called the combined test.
The most accurate screening test is non-invasive prenatal testing (NIPT), which you can have privately. It has a detection rate of 99 per cent. NIPT has a false positive rate of 0.1 per cent, which means it gives fewer false-positive results than the combined test.