Women are given the choice to just have the nuchal translucency, or to have the nuchal translucency as well as the blood test. However, the blood test cannot be performed on its own without the nuchal translucency. The blood test is best done before the nuchal translucency, but it is possible to have it done at the same time. Women carrying twins or more are not usually offered the blood test, because it is not as accurate.
How is a NT done?
Nuchal means 'neck'. The nuchal translucency (or NT) measures the depth of the fluid at the back of the baby's neck by using ultrasound. An ultrasound uses sound waves to create a visual black and white image of the baby. It is performed by placing an ultrasound probe (or 'transducer') on the woman's lower abdomen (called a 'transabdominal ultrasound') and conducting jelly on the abdominal skin to help the transducer create the image. You need to have a full bladder at the time of the test, this helps to lift the uterus up out of the pelvis to better view the baby.
Image 66-20 shows how the ultrasound is performed for a nuchal translucency.
Staff who perform nuchal translucency need to be specially trained. The more experienced the operator, the more likely the test will be accurate. The measurement is very small and must be carefully taken. It can take up to 30 minutes for the ultrasonographer to obtain an adequate measurement. If the baby arches their neck, this can abnormally increase the measurement, or if the baby lies with their back close to the wall of the uterus, the amniotic sac (encasing the baby and fluid) can be mistaken for the baby's skin and an incorrect measurement can be taken. Therefore, if the baby is lying close to the wall of the uterus, the ultrasonographer needs to wait for the baby to move away before taking the measurement.