Combined / Oscar test / First Trimester Screening Test
Choosing which tests to have in your pregnancy is an important decision for you and your baby
Why do I need an ultrasound scan?
There are several reasons why we offer an ultrasound scan to all women between 11-14 weeks of pregnancy.
- See the baby’s heart beating.
- Check whether there is just one baby.
- Confirm how many weeks pregnant you are by measuring the baby.
- Check there are no major abnormalities.
- Offer screening for Down syndrome
What is Down Syndrome and why is it relevant to me?
Down’s syndrome is the most common cause of severe learning disability in children and is often associated with physical problems such as heart defects (40%) or difficulties with sight and hearing.
All of us have 23 pairs of chromosomes and it is caused by the presence of an extra chromosome number 21 in the cells of the developing baby. It is not a disease or a hereditary condition (it is not passed down through generations of a family), it is caused by chance at conception. This means that any baby can be affected even if there is no family history of Down’s syndrome.
It can occur at any age and is more common with older mothers.The extra chromosome cannot be removed from cells, so there is no cure for the condition. As yet we do not know what causes the extra chromosome 21 to be present, it can come from either a mother or a father.
|Age (yrs)||Risk of Down’s Syndrome|
|20||1 in 1500|
|30||1 in 900|
|35||1 in 350|
|40||1 in 100|
|44||1 in 30|
Should I have the tests for Down’s syndrome?
Only you can decide that. Some women want to find out if their baby has Down’s syndrome,and some do not. Information about the tests and how they work can help you make up your mind.
Will the tests tell me for certain if my baby has Down’s syndrome?
We begin by offering all women a screening test (that carries NO risk of miscarriage).Screening test help to tell us which babies have an increased risk of having Down’s syndrome. We then offer diagnostic tests that give definite information to the women at increased risk. The problem is that having a diagnostic test carries a small risk of miscarriage. This is why we only offer a diagnostic test to women with an increased risk.
When is the test performed?
The combined test is performed between 11 weeks and 13weeks + 6 days. (length of the baby between 45- 84 mm)
What does the screening test involve?
There are two parts to the screening test; the first is a blood test. This measures the amount of two chemical substances that are found in the mother’s blood that have been passed from the baby. This can help to determine your risk of having a baby with Down syndrome.
The second part is an ultrasound scan to measure the thin layer of fluid under the skin at the back of the baby’s neck. This is known as the nuchal translucency (NT). Babies with Down syndrome can have an increased amount of fluid in this area. The results of the nuchal translucency, along with the blood test are combined with your age, weight and stage of your pregnancy to estimate the RISK of your baby having Down syndrome. A risk can also be calculated for twin pregnancies.
Please note: Having a thickened nuchal translucency (NT) can be associated with other fetal anomalies, such as heart defects which will also be discussed in detail.
Can I do only the NT scan?
Combining the results of the scan and blood test gives a higher detection rate than the scan or blood test alone. The combined test identifies about 9 out of 10 women who have pregnancies with Down syndrome. This is equivalent to a 90% detection rate. The scan alone may detect only about 70% pregnancies with Down Syndrome.
What is a higher risk result?
If your screening result is above 1:250 cut-off figure we will offer you further tests.If the screening test shows the risk of your baby having Down syndrome is lower than the cut-off figure, we will not offer you further tests. Most screening test results fall into this category.
This is known as having a LOW RISK or SCREEN NEGATIVE result. It does not mean that there is no risk at all that your baby hasDown’s syndrome, just that it is unlikely.
What happens next if I have a higher risk result?
Overall about one in 33 (3%) women screened have a higher risk result and are offered a diagnostic test.
You can decide not to have a diagnostic test.
This carries no risk to your pregnancy but leaves some uncertainty until after your baby is born.
You can decide to have a diagnostic test.
This will give you a definite result but carries a small risk of miscarriage. You need to think carefully about what you would do if you found yourself in this position. If you would not be happy with either of the above options, we can offer you genetic sonogram (second trimester blood test screening and anomaly scan that can help you make a decision before 20 weeks)
Diagnostic tests for Down’s syndrome
There are two diagnostic tests for Down’s syndrome:
- Chorionic villus sampling (CVS) &
A CVS is offered in early pregnancy from 11 weeks and involves taking a small sample of placental tissue (using local anaesthetic) by inserting a needle through your abdominal wall.
It is performed under ultrasound guidance. The result is normally available from 2-3 weeks. About 0.5% ( 1:200) of women who have a CVS will have a miscarriage.
An Amniocentesis can be performed from about 16 weeks of pregnancy and involves taking a sample of amniotic fluid from around the baby by inserting a needle through your abdominal wall. It is performed under ultrasound guidance to ensure the needle is a safe distance from your baby.The results can take up to 3 weeks. As with a CVS there is a risk of miscarriage of about 0.5% (1:300). Whichever invasive test you may choose (depending on the stage of your pregnancy), if the results from the invasive tests are negative it will rule out Down’s syndrome.
What are the possible results from diagnostic tests?
Your baby does not have Down’s syndrome. This is the most common result.
Your baby has Down’s syndrome
A very small number of women will get this result.
What happens if my baby has Down’s syndrome?
There are then two options; it is entirely the parents’ decision which one they choose.
Some parents will decide to continue with the pregnancy, make plans and prepare for any extra challenges they might face bringing up a child with Down’s syndrome.
Other parents decide they do not want to continue with the pregnancy and will choose to have a termination.