Introduction:
NT scan also called Nuchal Translucency scan is a non-invasive ultrasound scan done on pregnant women between 11-13.6 weeks gestational age to detect risk of chromosomal abnormality.
How is the test done?
The test is performed when the fetus is 11 weeks to 13 weeks 6 days old. The test is performed using a high end ultrasound scanner machine which uses sound waves to form images of the fetus. The scan is done by a Radiologist or sometimes by the gynaecologist or trained sonologist. The image needs to be very precisely taken and multiple criteria are taken to ensure that the image is optimal. The skin in the back of the neck forms a lucent band on the scan image and this is measured. Other structures such as the nasal bone are simultaneously looked for.
What is the use of the test?
The test is done to ensure that the baby born is free of chromosomal defects such as Downs syndrome, Turner syndrome, Trisomy of 18th and 13th chromosome and Triploidy. While no test can give a 100% guarantee, this test along with few blood tests (triple marker test), can give a reasonable detection rate for these problems.
Who is the scan for?
There are certain high risk populations where there is increased chance of chromosomal anomalies. These include maternal age aver 35 years, and previous fetus with abnormality. However in practice even young mothers can undergo these tests as even they have a small risk(1 in 1400) of having children with chromosomal abnormality, and the benefit of knowing far far outweighs the cost of the test.
How accurate is it?
Nuchal scanning alone detects 62% of all Down's Syndrome with a false positive rate of 5.0%; the combination with blood testing gives corresponding values of 73% and 4.7%.
When screening is positive, Chorionic Villus Sampling (CVS) or amniocentesis testing is required to confirm the presence of a genetic abnormality. However this procedure carries a small risk of miscarriage so prior screening with low false positive rates are needed to minimize the chance of miscarrying.