NT + Blood test
How is a NT done?
What will a NT tell me?
Nuchal (pronounced 'new-cal') translucency or 'NT' is a relatively new screening test. It uses ultrasound to visualise and measure a fluid filled sac at the back of the unborn baby's neck during early pregnancy. NT can be done as early as 10 weeks + 3 days of pregnancy, but it is more accurate when performed after 11 weeks + 3 days and before 13 weeks + 6 days. After 14 weeks, the lymph system of the baby develops, making the fluid naturally drain away, meaning the test can no longer be done.
Nuchal translucency is used to estimate if a baby is at increased risk of having a chromosomal abnormality. The main genetic disorder screened for is Down syndrome (also called 'trisomy 21'). People with Down syndrome have an extra chromosome. This can make the person have specific physical features and various levels of intellectual disability and possibly other physical problems. However, many people with Down syndrome lead active and full lives in semi-dependent manner. Other extremely rare genetic disorders that may be detected are Edward's syndrome (trisomy 18) and Patau syndrome (trisomy 13).
NT + Blood test
When a woman is offered a nuchal translucency, she will also be offered a blood test. A nuchal translucency alone is about 75% accurate, but if it is combined with the blood test the accuracy can be increased up to 85%. The blood test measures the level of two proteins in the woman's blood. These are called 'pregnancy associated plasma protein - A' or 'PAPP-A' and 'free-beta human chorionic gonadotropin' or 'free BHCG'.
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.
Occasionally, the position of the baby makes visualising the back of their neck very difficult. If this is the case, the ultrasonographer may ask you if they can perform the ultrasound vaginally (called 'transvaginal'). This means the ultrasound probe is covered by a plastic sheath, similar to a condom and gently placed inside the vagina (but outside the cervix). The 'transvaginal' ultrasound enables the ultrasonographer to look more closely at the baby.
NOTE:Most women do not expect that their nuchal translucency could involve placing something into their vagina. A few women will feel very strongly about not having their nuchal translucency done in this manner. Be aware that you are not obliged to consent to it if you do not feel comfortable about having a transvaginal ultrasound. You may opt to have the abdominal ultrasound repeated a few days later, or decline to continue with the test all together.
What will a NT tell me?
Generally, the larger the measurement (or depth of fluid), the greater the chance the baby may have a genetic disorder. However, there is no 'normal' measurement because it varies widely between babies, and can even vary for different pregnancies in the same woman. The measurements are also interpreted in relation to each woman's age. As a guide, the 'top' level of normal tends to be somewhere between 2 to 3 millimetres (mm), with above 3mm definitely being larger than normal.
The nuchal translucency measurement (+/- the blood test results), along with the woman's age and the estimated age of the baby (measured by the ultrasound) are combined to calculate a 'risk figure', or the chances of the baby having a genetic disorder. The result may be given as just 'low risk' or 'high risk' or you may be given the neck measurement and a specific figure expressed as an 'odd'. A 'low risk' is considered more than 1:300 (for example, 1:400), a 'high risk' is considered less than 1:300 (for example, 1:30). These estimates are explained fully in class B, understanding 'the numbers'.
A nuchal translucency is only a screening test, it aims to identify babies that may be at increased 'risk' (or have a higher chance) of having a genetic disorder, but it cannot definitely tell if the baby does have a genetic disorder. Nuchal translucency alone is about 75% accurate, but if it is combined with the blood test, this increases the accuracy up to 85%. This means that around 15 to 25% of babies with a genetic disorder such as Down Syndrome will be missed, meaning the woman will be estimated as 'low risk' and yet still carry a baby with a genetic disorder. About 5% (or 1:20) women will be estimated as 'high risk', yet most of these babies will be very healthy and normal (this is called a 'false positive result').
NOTE:Despite the level of inaccuracy in all screening tests, a nuchal translucency + the blood test is widely accepted as generally being the most accurate in predicting affected babies, especially when compared to the triple blood test, which was more popular before the use of NT.
The results of a nuchal translucency may be given immediately after the test or a letter may be sent to your caregiver. If you have the blood test at the same time, the results from the blood test can take several working days. If your nuchal translucency result is 'high risk' (or less than 1:300), you will be contacted and offered further genetic counselling to discuss the options of having diagnostic genetic tests such as CVS or amniocentesis. It is your choice as to whether you accept these tests. If an amniocentesis or CVS shows your baby has a genetic disorder, you have the choice to continue the pregnancy or terminate the pregnancy. There are no cures or treatments.
Whatever your decisions, it is important that you have further counselling and receive support and adequate time to make the choices that are right for you.
Last revised: Monday, 12 November 2012
This article contains general information only and is not intended to replace advice from a qualified health professional.