Nuchal Translucency Scan
Traditional Screening for Downs Syndrome
Various tests have been developed to try to identify those women at greatest risk of having a chromosomally abnormal baby such as Downs Syndrome who could then decide whether or not to proceed to amniocentesis. Originally all women over 35 were offered an amniocentesis but this only detected about 30% of all babies with Downs Syndrome. The Triple Test at about 15 weeks into the pregnancy detects about 65% of Downs Syndrome pregnancies based on about 1 in every 20 (5%) women having the blood test ending up with a high risk result (i.e more than 1 in 250) and then proceeding to amniocentesis.
Disadvantages of Serum Screening (Triple Test)
The test is less efficient in younger women with a detection rate of only about 30% and in older women over 35 there is a high likelihood that testing will place them in a high risk category warranting amniocentesis.
The blood test is not done until 15 weeks and therefore decisions about amniocentesis are made at 16 weeks with final results and decisions about the pregnancy made at 18 weeks. At this late stage the woman can feel the baby’s movements and the pregnancy is almost half way through. If termination is considered for an abnormal result it is usually by a medically induced birth which may mean up to 2 to 3 days in hospital.
The test is not valid for women with multiple pregnancy and cannot be relied upon in women who have had a previous baby with a chromosomal abnormality such as the common type of Down’s syndrome (Trisomy 21).
If all is well then the woman will have had to wait until nearly 18 weeks into the pregnancy to have the reassurance that the baby does not have Down’s Syndrome if her blood test shows a high risk result.
Nuchal Translucency Scan
A new test based on an ultrasound scan at about 12 weeks has now been evaluated in studies of over 200,000 women and found to have a high detection rate across all ages. The centre is licensed and quality controlled by the Fetal Medicine Foundation in London who have pioneered this new approach to screening. All scans are performed by a Consultant Obstetrician.
By measuring the thickness of tissue fluid that lies under the skin at the back of the baby’s neck, the Nuchal Translucency, a computer programme can convert this, the length of the baby and the mothers age into a risk for Down’s Syndrome for that baby. Because it is based on the appearance of an individual baby, the technique can also be used in twins and triplets etc, to give a risk for each baby. The test has a detection rate of 75% based on scan alone rising to nearly 90% in single pregnancies if blood markers (PAPP-A and free B-hCG) are also measured in a sample of blood taken from the pregnant woman’s arm at the time of the scan. If the hCG is high and the PAPP-A is low this increases the risk of Down’s Syndrome.
A Nuchal translucency scan can be performed between 11 weeks and 13 weeks and 6 days.
Nasal Bone
The nasal bones are paired bones which go on to form the bridge of the nose. If the nasal bone is absent the risk of Down’s Syndrome is increased. The nasal bone can sometimes be very difficult to see. The fetus has to be in exactly the right position and to see clearly the fetal head needs to be in the correct angle. We like to see the nasal bone wherever possible but if the nuchal translucency and blood tests are predicting a low risk result it is not necessary to see the nasal bone in every case.
The main benefit of using the nasal bone is to reduce the ‘false positive rate’. This means that fewer women will be given a high risk when their baby is normal. Until now the screening tests for Down’s Syndrome have given about 5% of women a risk higher than 1 in 250, but with the inclusion of nasal bone that figure is reduced to 3%. This means that fewer CVS & amniocentesis tests are needed, and fewer miscarriages are caused. At the same time there is a slight increase in the detection rate if the nasal bone is used.
How do I get my results?
We either give you the results immediately after the scan, or if you are having the blood test on the day of the scan we will phone you when the results come back, usually within 2 working days. If you get a high risk result arrangements will be made to discuss this risk in person.
How do I arrange a test?
To make an appointment please call Thornbury Hospital, Sheffield on 0114 267 4408 Monday – Friday, during working hours.