The OSCAR test is offered to women between 11 and 14 weeks of pregnancy, to assess the risk of their baby having Down's syndrome. Our Clinic bases this assessment on a combination of tests, including a nuchal translucency scan measurement (NT) and a blood test. The entire procedure usually takes about an hour and thirty minutes. You receive the full result on the day, and can discuss this with the specialist performing the scan. Bristol Pregnancy scans provide the only one-stop OSCAR service in the South West of England.
The assessment is based on tests that consider the following factors:
The combination of these tests used at the OSCAR is a form of screening. This means the tests identify women with an increased risk of having a baby affected by Down's syndrome (trisomy 21), or certain other chromosomal abnormalities such as trisomy 13 or 18. However, it is not a diagnostic test and so cannot give a definite yes or no result.
Women identified as being at high risk by the OSCAR are offered a separate diagnostic test. This is either amniocentesis or chorionic villus sampling, which can be performed by one of our fetal medicine consultants. These tests determine the chromosome count of the baby, and will exclude or confirm Down's syndrome.
The combination of tests is able to correctly identify around 90% of high-risk pregnancies.
Screening tests, such as the OSCAR test, help define a small percentage of the total population, called 'high risk', in which the majority of the Down's babies will be found. For the OSCAR test, about 4% of those screened will be found to be 'high risk', and this group will include nearly 90% of the Down's babies.
We are often asked what the difference is between an OSCAR test and the First Trimester Screening Test (FTS) offered by some NHS trusts. Here is a chart outlining the differences:
|NHS First Trimester Combined Screening (FTCS)||One stop clinic for assessment of risk (OSCAR)|
|Blood test and scan done on same day||Yes||Yes|
|Timing of results||
||Immediate for all women|
|Format of results||
||Risk ratios of all 3 syndromes calculated and reported immdiately|
|Discussion of high risk results||By telephone with antenatal clinic midwife. Appointment then made for discussion with fetal medicine midwife / consultant on a separate day||Immediate, face to face, with consultant / midwife|
|Choice of appointment times||No||Yes|
|Evening and Saturday clinics||No||Yes|
|Scan pictures||Pay extra||Lots of pictures included, with time available for the best quality images.|
|Pictures provided on USB/memory stick, so possible to send on Email, facebook etc. immediately.||No||Yes, no extra cost|
The results of all of the measurements are entered into a computer program to produce an overall assessment of the risk of the pregnancy being affected by Down's syndrome. This assessment starts with a calculation of your background risk, which applies to every woman of the same age and at the same stage of pregnancy as you. This background risk is then modified using the extra information from the blood test and the ultrasound scan to give a specific risk for your current pregnancy (the adjusted risk).
The risk assessment relies on accurate knowledge of the stage of pregnancy. If either you, your midwife or the consultant has any doubt about your dates, we recommend you arrange to have a dating scan performed before attending the OSCAR, so that the risk assessment is timed for the correct stage of pregnancy.
A consultant obstetrician or midwife specialist in ultrasound, who has completed the Fetal Medicine Foundation Course, will perform your scan, and calculate your risk assessment using all the information gathered.
When you arrive, a nurse will take a blood sample from a vein in your arm. You will also be weighed.
The blood sample is used to measure your blood levels of a hormone (HCG) and a protein (PAPP-A). These are produced by the placenta (the afterbirth) and the womb. The levels of these substances gives important information about the risk of Down's syndrome.
Analysis of the blood tests in the laboratory takes approximately 30 minutes. If it is a pleasant day, we suggest you leave the clinic and take a short walk. Local cafés can be found just 5 minutes from our clinic, or the shops of Whiteladies Road, or a stroll on Bristols’ famous Downs might appeal. Once the blood test analysis is complete the specialist will collect you to have your ultrasound scan.
The assessment is usually performed by scanning through your tummy. You do not need a full bladder. Depending on the position of your baby and your womb, sometimes it's necessary to perform an internal (trans-vaginal) scan. This involves an ultrasound probe being placed in the vagina. There is no risk to the pregnancy and most women find this scan less uncomfortable than a smear test.
The result will express the probability of the baby having Down's syndrome.
"Low risk" is a result where the risk is less than one in one hundred and fifty (1:150). On your results print-out this will be expressed as a ratio, for example, 1:500. This means that out of 500 women who have this level of risk, one will have a baby with Down's syndrome.
"High risk" is a result where the risk is greater than 1:150, for example 1:100. This means that out of every 100 women who have this level of risk one will have a baby with Down's syndrome.
The scan will also identify twins, confirm the expected date of delivery and exclude some major abnormalities. However, it is also important to have a detailed scan at 18-20 weeks, as the fetal organs are more developed, and can therefore be seen more clearly at this stage.
For those in the high risk group (greater than 1:150), the only way to be certain that the pregnancy is not affected by Down's syndrome is to have an amniocentesis or CVS (chorionic villus sampling). These diagnostic tests involve an invasive procedure to sample the fluid that surrounds the baby (amniocentesis) or the placenta (CVS). Both carry a risk of causing a miscarriage. The risk of miscarrying what may be a normal baby must be considered against the risk of the baby having Down's syndrome.
You will be given a copy of the result, and scan pictures of your baby, which you can take home. The consultant or midwife sonographer will discuss your result with you, and talk through the options available to you if you have a high risk or borderline result. They will answer any queries or concerns that you may have.
You should experience no physical side-effects from the test and should be able to leave the centre shortly after your scan.
If you wish to speak to one of the OSCAR team, please telephone on 0117 980 4070.
If you want to find out more about Dowin's, Edward's and Pataus syndromes or feel you need further support, contact websites and telephone numbers are listed below.