Nuchal Translucency Scan is an early pregnancy scan which is performed between 11 to 14 weeks. The 6 weeks pregnant scan generally confirms a live intrauterine fetus. The next scan that you will be advised for is a NT scan which is a first trimester screening test for Down’s syndrome.
Nuchal Translucency is the measurement of the translucent fluid collected at the back of the baby’s neck. This fluid can be measured on ultrasound and the values can be interpreted.
How is the Nuchal Scan Performed?
All pregnant ladies are offered this scan between 11 to 14 weeks of their pregnancy. It is best done when the CRL (crown rump length) of the baby measures between 45 mm to 84 mm. The scan may be done alone or combined with a series of blood tests which are Triple Marker Test or Quadruple Test.
The Triple test measures the levels of three important markers secreted by the placenta –
- Alpha fetoprotein (AFP)
- Human Chorionic Gonadotropin (HCG)
Quadruple test measures the above three in addition to Inhibin A
NT Scan is an abdominal ultrasound scan. The sonographer will first apply gel on your tummy and spread it with the help of a hand held probe (transducer). The ultrasonic rays from the transducer will hit the uterus and get reflected back. These are then converted by the computer into visual images. The images are seen as black and white moving objects. You may feel some pressure as the examiner tries to get a better view or get to the exact location that is the back of the baby’s neck. Sometimes if the uterus is tilted backwards, in overweight patients or if the baby is in an odd position, a transvaginal scan may be performed by inserting a long cylindrical probe covered with a condom and lubricated with a jelly into the vagina.
First, the baby will be measured from head to toe. This is called as CRL (crown rump length) which will give the age of the baby in weeks. After that the back of the neck will be located and the examiner will measure a black space which is the fluid collection. The skin will be seen as a white line. The measurement will be recorded in “mm”.
In this scan you will also be able to see the various body parts – head, spine, limbs etc and the baby moving his hands and feet. Major structural abnormalities can be picked up but it is still too early to know about the baby’s complete development. A detailed study of the baby’s anatomy will be done during the 3d ultrasound or the 4d ultrasound, medically known as the Anomaly Scan which is done between 20 to 22 weeks of pregnancy.
These scans (the 12 week scan and the 20 week scan) are routinely performed at the antenatal clinics or at hospitals by certified sonographers having registered sonogram machines.
Interpretation of the Nuchal Translucency Scan
NT values are best interpreted when measured between 11 to 14 weeks of pregnancy with the baby’s CRL ranging from 45 to 84 mm.
- Normal values are generally below 3.5 mm. Most of the babies have an NT between 2.5 to 3.5mm
- The values of NT increase as the baby grows. So the test results are insignificant if done after 14 weeks.
- NT values as high as 5-6mm are an indicator of risk for Down’s syndrome and other genetic disorders.
The risk of the baby having Down’s syndrome is primarily related to the age and family history of the mother. So the mother’s age is an important part of the assessment criteria. The age of the mother and the NT values are fed into a database which gives a fairly accurate risk analysis. High risk cases have a risk ratio of 1:150 or less. However it is seen in some cases that even mother’s falling in the high risk category have given birth to normal babies.
Prognosis of High Risk Cases
In case you fall in the high risk group, you will be referred to a Fetal Medicine Unit which is made up of specialized doctors and nurses. Here, further workup will be done to confirm the presence of any anomaly in the baby. This is done by CVS (chorionic villi sampling) and Amniocentesis.
The team will provide you guidance and counseling about the health of the baby and whether the pregnancy should be continued, risks involved etc.
The accuracy of NT scan is 77 percent in picking up Down’s syndrome. Combining it with the screening blood tests is known to give more accurate results.