Nuchal translucency is a collection of fluid under the skin at the back of your baby’s neck. All babies have some fluid at the back of their neck. But many babies with Down’s syndrome have an increased amount.
A nuchal translucency (NT) scan is a screening test that assesses whether or not 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 tell you for sure. However, diagnostic tests do carry a small risk of miscarriage.
An NT scan can’t tell you for certain whether or not your baby is affected. What it can do is help you to decide whether or not to have a diagnostic test.
1- 1 How is the NT scan performed?
This ultrasound scan must take place:
- Between 11 weeks and 13 weeks plus six days of pregnancy
- When your baby measures between 48mm and 85mm
Before this the scan is difficult to do because your baby is still so tiny. It would also be too early to combine it with your first trimester blood test. After 14 weeks, any excess fluid may be absorbed by your baby’s developing lymphatic system.
The scan is usually done through your tummy. However, a vaginal scan may be needed, as it gives better views, especially if your uterus (womb) tilts backwards, or if you are overweight.
To date your pregnancy accurately, the 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.
What is a normal measurement?
An NT of less than 3.5mm is considered normal when your baby measures between 48mm and 85mm. The NT normally grows in proportion with your baby. However, an increased NT does not mean there is definitely a problem. Some babies without Down’s have increased fluid, too. As the NT increases, so does the risk of Down’s and other chromosomal abnormalities.
How accurate is the NT scan?
The NT scan picks up about 75 percent 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 they don’t actually have it. This is called a false positive. The false positive rate for an NT scan is five percent.
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 percent.
This scan requires a high-resolution ultrasound device and an experienced examiner. The FETAL MEDICINE FOUNDATION Germany (FMF-Germany) trains colleagues in this method. Dr. Maria Rastgou has obtained the certificate of using this device under supervision of FMF.