What is Nuchal Translucency (NT)
Nuchal Translucency (NT)
You probably heard the above term mentioned in the context of pregnancy scans, but what does it actually mean? Nuchal Translucency is a sonographic phenomenon which can be seen best at around 11-13 weeks of gestation. It refers to the thickness of the liquid that can be seen on an ultrasound image behind the baby’s neck.
Why is it important? Research has shown that an increased NT thickness can indicate a higher chance of occurrence of certain chromosomal or structural anomalies. The key word being CAN; in some cases the excess fluid will dissolve further along the pregnancy and the babies will be born completely normal.
The NHS cut off for normal NT thickness measurement is anything below 3.5mm. It is measured as part of the Combined Screening where a blood sample as taken as well to assess the chance of 3 chromosomal anomalies; Down’s Syndrome, Edward’s Syndrome and Patau’s Syndrome. Being a screening test, it is not diagnostic meaning that it cannot give a definite yes/no answer. In case of higher chance results, further investigation is required.
More recently, a new screening test known as Non Invasive Prenatal Test (NIPT) was developed. It involves a simple blood sample taken from the mother’s arm and is capable of detecting Down’s Syndrome with a 99% accuracy, vs 76% accuracy of the Combined Test and 69% accuracy for the Nuchal Test alone. You should take these percentages with a pinch of salt, given Down’s Syndrome in itself is a reasonably rare anomaly which is becoming more likely with age; 1:1250 for a 26-year-old pregnant woman rising to 1:100 for a 40-year-old.
NT can be a useful indicator for certain anomalies, but should be used very carefully for screening purposes. An increased NT measurement warrants a further investigation such as examination of the baby by ultrasound to exclude certain structural anomalies (especially heart defects) and possibly diagnostic invasive tests such as CVS or Amniocentesis. These invasive tests carry a small risk of miscarriage; an alternative to avoid these invasive test is screening using NIPT such as the Harmony Test.
At City Ultrasound we also perform an examination of the fetal heart (echocardiography) with every scan from 12 weeks, to screen for potential heart defects.
Please note – ultrasound itself cannot exclude all anomalies/complications that can occur during pregnancy.
In the video, Dr Ushakov demonstrates how the NT is measured using ultrasound. This video from 2011 has ~370,000 views on YouTube. Read more about Nuchal Translucency on our website.