Nuchal translucency (NT) ultrasound

What is a nuchal translucency ultrasound measuring?

Nuchal translucency is a collection of fluid under the skin at the back of a baby's neck. Nuchal translucency (NT) can be measured using ultrasound when your baby is between 11 weeks and 13 weeks plus six days old. All babies have some fluid, but many babies with Down's syndrome have an increased amount.

A nuchal translucency (NT) ultrasound is a screening test which assesses whether your baby is likely to have Down's syndrome. A screening test can only estimate the risk of your baby having Down's, whereas a diagnostic test, such as CVS or amniocentesis will give you a definite diagnosis (but also carries a small risk of miscarriage).

Combining an NT ultrasound with a blood test, often called a triple test or a combined test, gives a more accurate result. The blood test measures the levels of the hormone free beta-hCG and a 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 ultrasound is combined with this blood test, the detection rate improves to about 90 per cent. This test is called the combined test. Read our article on screening for Down's syndrome to find out more about this and other screening tests.

The NT ultrasound can't tell for certain whether your baby is affected. However, it can help you decide whether or not to have a diagnostic test.

How is the NT ultrasound performed?

A NT ultrasound must be performed between 11 weeks and 13 weeks plus six days of pregnancy. Before 11 weeks the ultrasound is technically difficult because the baby is so tiny and, after 14 weeks, any excess fluid may be absorbed by the baby's developing lymphatic system.

The ultrasound is usually done through your tummy, but occasionally it's necessary to have a vaginal ultrasound, which will give better views. There is no risk to you or your baby and it should not be too uncomfortable. Read about vaginal ultrasound in our overview of ultrasounds .

To accurately date your pregnancy, the person performing the ultrasound, called a sonographer, will measure your baby from the top of his head to the bottom of his spine. She will then measure the width of the NT. The skin will appear as a white line, and the fluid under the skin will look black.

You will be able to see your baby's head and spine, limbs, hands and feet on the screen. Some major abnormalities may be excluded at this ultrasound, but it's recommended you also have a mid-pregnancy ultrasound between 18 and 22 weeks.

It's a good idea to take your partner or a friend with you, to share the experience. You will not likely get any results until your next prenatal appointment. Most sonographers are not allowed to share results with you. The ultrasound must first be interpreted by a radiologist who will then send your results to your doctor or midwife.

What is a normal measurement?

An NT measurement of up to 2mm is normal at about 11 weeks, and up to about 2.8mm by 13 weeks and six days. The NT normally grows in proportion with your baby. The image, left, shows a baby with a normal NT of 1.3mm; this baby will be low-risk. An increased NT does not mean there is definitely a problem. Some babies without Down's have increased fluid, too. The image, right, shows a baby with an NT of 2.9mm, which is at the upper limit of normal range. Nine out of 10 babies with a measurement between 2.5mm and 3.5mm will be completely normal.

As the NT increases, so does the risk of Down's and other chromosomal abnormalities. The baby in the image, left, has an NT of 6mm and has a high risk of Down's, as well as other chromosomal abnormalities and heart problems. Fortunately, not many babies have as much fluid as this.

How is the risk of Down's calculated?

Every woman has a risk of giving birth to a baby with Down's syndrome, and this risk increases with age.

To calculate your risk, the sonographer will enter your age and the measurements from your NT ultrasound into a database. It shows the risk that applies to all women of your age, known as the background risk.

Your ultrasound measurements will be combined with your age to generate your own individual risk for this pregnancy. This may be higher or lower than your background risk.

What should I do if I am high risk?

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, your baby has four out of five chances that he does not have Down's. Nevertheless, once your pregnancy has been labelled high-risk, you might feel anxious and perhaps unsure of what to do.

The only way to know for certain if your baby has Down's or another chromosomal abnormality 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 ultrasound is that it is done early in your pregnancy, and it is possible to have a CVS and get the result while you are still in the first trimester. If you are unsure what to do, you can wait until 16 weeks and have an amniocentesis then if you wish. Your midwife or doctor will provide more information if you need it.

Will I be given a risk for any other abnormalities?

You may also be given a risk for two other chromosomal abnormalities, Edwards' syndrome and Patau's syndrome. These conditions are much rarer than Down's, and most affected pregnancies miscarry. Babies with these conditions usually also have abnormalities that can be seen on the ultrasound.

How reliable is a nuchal translucency ultrasound?

A large research study involving over 100,000 pregnancies found that about 75 per cent of babies with Down's syndrome were correctly identified using this method. When combined with a blood test, the detection rate improves to about 90 per cent. However, to achieve these detection rates, it is important that the NT is measured accurately. For this reason, strict standards have been devised so that every sonographer measures in exactly the same way. To use the database to calculate the risk, sonographers must first undergo training by Fetal Medicine International and, once they are considered competent, their technique and results are reviewed every year.