The Nuchal Translucency scan (NT) is an ultrasound performed between 11 weeks 3 days and 13 weeks 6 days in conjunction with a blood test. However, we try to organise your appointment for between 12 weeks 4 days and 13 weeks 2 days. This means that should your baby measure a little small for dates, you do not have to come back to have the scan repeated. Alternatively, if your baby is big for dates, we can still undertake the assessment. It is important to note that the NT scan is a screening test, not a diagnostic test. The nuchal translucency is a fluid filled collection located at the back of the fetus’ neck. All fetuses have this fluid collection and it is well visualised towards the end of the first trimester. We know that a fetus with Down syndrome will usually have a much larger fluid collection than a fetus that does not have Down syndrome.
When a woman is pregnant, there are two hormones she produces that may vary if a baby has Down syndrome. The levels of these hormones are measured ideally between 10 and 11 weeks gestation, prior to the ultrasound being performed (if possible). If they are found to be in the normal range, this decreases the chance of the baby having Downs syndrome. The hormones that we test are PAPP-A and free beta HCG. The level of PAPP-A tends to decrease when the baby has Down syndrome, while the level of free beta HCG tends to increase.
Once the nuchal translucency is measured and the scan is complete, the sonographer will enter the information from the scan and the blood test into a computer programme which will assess the risk for this particular fetus.
The results would be given to you as a chance of the fetus having a chromosome anomaly. Please see chart below.
If you have missed the opportunity or choose not to have a Nuchal Translucency Screening scan a first trimester scan can still provide an opportunity to check the baby for many fundamental abnormalities such as limb defect, major heart defects etc.
Patient attends NT screening
test at 12- 13 ½ weeks
Moderate risk aneuploidy 1:100 – 1:1000
Low risk aneuploidy
Better than 1:1000
High risk aneuploidy
1:2 – 1:100
Amniocentesis or CVS
Fetal DNA testing
at 19 – 20 weeks
Morphology Scan at
19 - 20 weeks