What is nuchal translucency?
Nuchal translucency (NT) is a collection of fluid under the skin at the back of a baby's neck. The nuchal translucency scan measures the thickness of this fluid to assess the risk of Down's syndrome. It can be measured using ultrasound.
If the scan reveals that there is more fluid than usual, it could be a sign of Down's syndrome. Another sign of Down's syndrome is the shape of the baby's nasal bone. Your doctor will order another scan to measure the nasal bone if he suspects signs of Down's syndrome.
Why might I have a nuchal scan?
All pregnant women are offered a nuchal translucency (NT) scan, to assess whether or not their babies are likely to have Down's syndrome. It is part of the detailed scan you will get in your first trimester.
A nuchal translucency scan can only indicate the level of risk of your baby having Down's syndrome. It can't tell you for sure whether your baby will have the abnormality or not. It is what is known as a screening test as opposed to a diagnostic test that gives you an exact diagnosis.
For example, if your result is one in 1,000, this means that for every 1,000 babies with your level of risk, one will have Down's syndrome. A risk of one in 150 or less is considered high.
Diagnostic tests such as chorionic villus sampling (CVS) or amniocentesis are recommended only if the screening test indicates that you are at a high risk of carrying a baby with Down's. However, these tests also carry a small risk of miscarriage.
The NT scan, along with a blood test that is part of the standard Down's screening, will give you as much information as possible, so you can decide whether you want to have a diagnostic test.
When will I have the nuchal scan?
An NT scan must take place at a particular time in your pregnancy. It is done between 11 weeks plus two days and 13 weeks 6 days when your baby's crown rump length (CRL) is between 45mm (1.8in) and 84mm (3.3in).
Before 11 weeks the scan is technically difficult because your baby is so tiny. It would also be too early to combine it with the blood test that screens for Down's.
After 14 weeks, any excess nuchal fluid may be absorbed by your baby's developing lymphatic system. So the test would not be accurate.
How is a nuchal scan performed?
The scan is usually done through your tummy using normal ultrasound equipment. Like other abdominal scans done in your first trimester, you will need to have a full bladder.
The ultrasound doctor will put some gel on your tummy and move a handheld device (transducer) over your skin. It shouldn't hurt, though you may feel some pressure as the doctor 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 you'll be offered a vaginal scan. This is particularly likely if your womb (uterus) tilts backwards, or if you're overweight.
The doctor will start by measuring your baby from the top of his head to the bottom of the spine. This will help her to date your pregnancy accurately. She will 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.
Your baby fits nicely on the screen at this stage, and you will be able to see the head, spine, limbs, hands and feet. Some major abnormalities may be ruled out at this scan as well, such as problems with your baby's abdominal wall and skull. But a more detailed anomaly scan (ultrasound level II) will be prescribed between 18 and 20 weeks.
What is a normal nuchal translucency measurement?
An NT of less than 3.5mm is considered normal when your baby measures between 45mm (1.8in) and 84mm (3.3in). The NT normally grows in proportion to the growth of the baby. Image 1 shows a baby with a normal NT of 1.3mm; this baby will probably be low risk.
1. Normal nuchal translucency
However, an increased NT does not mean there is definitely a problem. Some babies without Down's have increased fluid, too. Image 2 shows a baby with an NT of 2.9mm, which is also within the limits of the normal range. Though the risk will be higher than the baby's in the first image, nine out of 10 babies with a measurement between 2.5mm and 3.5mm will be completely normal.
2. Borderline nuchal translucency
As the NT increases, so does the risk of Down's and other chromosomal abnormalities. The baby in image 3 has an NT of 6mm and will be at a very high risk for Down's, as well as other chromosomal and genetic syndromes and heart problems. Fortunately, not many babies have as much fluid as this.
3. Increased nuchal translucency
When will I get the results after the nuchal scan?
It usually takes a day for the ultrasound report to be ready. If you are getting your scan done at the same hospital in which your doctor practices, the report will be sent directly to your doctor. Your doctor will then discuss the results with you at your next appointment.
If you get your scan done at a diagnostic centre, you might have to go back the next day to collect the report and bring it with you to your next appointment. Sometimes, diagnostic centres deliver reports within a few hours but you may have to ask specifically for this.
What do the results of the nuchal scan mean?
The results will express the probability of your baby having Down's syndrome. They will be given in the form of a ratio. For example, 1:800 means that out of 800 women who have this risk level, one will have a baby with Down's syndrome.
Low risk is a result where the risk is less than one in 300 such as 1:500.
High risk is a result where the risk is greater than one in 300, for example, 1:150. This means that out of every 150 women who have this risk, one will have a baby with Down's syndrome.
How is the risk of Down's syndrome calculated?
Every woman has a chance of giving birth to a baby with Down's syndrome. The chance increases with your age. So even before you're pregnant, you will have a certain risk level, based on your age (background risk).
To calculate your overall risk, the ultrasound doctor will enter your age and the measurements from your NT scan into a database. Once your NT measurements have been taken into account, your risk may be higher or lower than your background risk.
What if I have a high risk of Down's syndrome?
Most women given a high risk (one in 150 or less) will go on to have a baby without Down's. Even with a risk as high as one in five, there's a four out of five chance that your baby doesn't have Down's. Nevertheless, once your pregnancy has been labelled high-risk, you will probably feel anxious and perhaps unsure of what to do.
The only way to confirm Down's is to have a diagnostic test such as CVS or amniocentesis. This decision can be very difficult, but you do not have to decide in a hurry.
One advantage of the NT scan is that it is done early in your pregnancy, which gives you plenty of time to decide your next steps. It's possible to have a CVS and get the result while you're still in your first trimester. If you're unsure, you can wait until 15 weeks and have an amniocentesis if you wish.
Your doctor will thoroughly explain these diagnostic tests. You'll also have the chance to talk about what any abnormalities could mean for the future health and development of your baby.
Some parents who discover that their unborn baby is affected by abnormalities may consider the difficult decision about whether to end the pregnancy. However, you should never be expected to consider ending your pregnancy based on NT scan findings alone because these just evaluate your risk, they don't tell you for sure if your baby has Down's. Always ask for a second opinion if you are in any way unhappy with the advice you're given.
Will I be given a risk for any other abnormalities?
You'll be given a risk for Edwards' syndrome and Patau's syndrome, which are two other chromosomal abnormalities.
These conditions are much rarer than Down's, and sadly most affected pregnancies do end in miscarriage. Babies with these conditions usually also have abnormalities that can be seen on the scan.
How accurate is the nuchal scan?
The NT scan picks up about 77 per cent of babies with Down's syndrome.
Sometimes, a screening test can suggest that a baby has a high risk of having Down's syndrome, even though the risk 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 in 20 women is wrongly given a high risk.
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 false positive rate for the Combined Test is also five per cent.
