DETECTION OF DOWN SYNDROME WITH ULTRASOUND AT 12 WEEKS OF PREGNANCY
1. Down syndrome is a congenital chromosome abnormality also called Trisomy 21. This means there is an extra chromosome 21 in every cell of the body.
2. Children with Down syndrome always have an intellectual and a physical disability.
3. All pregnant women of any age have a small risk of delivering a baby with Down syndrome. As women age, the risk of delivering a baby with Down syndrome increases.
Background Facts and Down syndrome:
1. It is possible to assess the risk of your fetus having Down syndrome with an ultrasound examination of the fetus at 12 weeks.
2. Fetuses with Down syndrome tend to have more fluid in the back of the neck than normal fetuses. The measurement of the fluid layer in the back of the neck than normal fetuses. The measurement of the fluid layer in the back of the neck is called the nuchal translucency.
3. About 80% of Down syndrome fetuses will have a nuchal translucency measurement that is thicker than most normal fetuses. However, some normal fetuses will have a thick nuchal translucency.
4. The scan alone cannot make a final diagnosis of Down syndrome. Even if the scan is normal it does not completely exclude the possibility that a fetus might be affected.
5. Most often the nuchal translucency is combined with a blood test to detect Down syndrome.
6. A definite diagnosis of Down syndrome requires an invasive needle test (Chorion Villus Sampling or an Amniocentesis). If your test shows a high risk for Down syndrome you will be offered one of these tests. These needle tests carry a risk of miscarriage of about 1 in 200.
What is your risk of having a baby with Down syndrome?
The figures below show how the risk of having a baby with Down syndrome increases with the mother’s age. These figures also show that the risk of a Down syndrome fetus is higher at 12 weeks than at birth. This is because many Down syndrome fetuses die naturally during the pregnancy (resulting in a miscarriage or still birth).
Maternal age related risks of having a live born child with Down syndrome and the risk of carrying an affected fetus at 12 weeks of pregnancy.
|Age (yrs)||Risk for trisomy 21|
|At Birth||At 12 weeks|
|20||1 in 1527||1 in 898|
|25||1 in 1352||1 in 795|
|30||1 in 895||1 in 526|
|32||1 in 659||1 in 388|
|34||1 in 446||1 in 262|
|36||1 in 280||In 165|
|38||1 in 167||1 in 98|
|40||1 in 97||1 in 57|
|42||1 in 55||1 in 32|
|44||1 in 30||1 in 18|
12 week scan
This is the best time in pregnancy to assess the risk of Down syndrome with an ultrasound examination. It is usually performed as an abdominal examination. It is usually performed as an abdominal scan, but sometimes it is necessary to do an internal (vaginal) scan. A full bladder is not needed but sometimes a little urine in the bladder can be helpful.
This scan will:
1. Confirm that the fetus is alive and has a heart beat
2. Check for multiple pregnancy – twins etc.
3. Assess the size of the fetus. This allows an accurate due date to be given. It also allows a
personalized risk of Down syndrome in your fetus. Risk calculation can only be made if your fetus is
between 45 and 84mm in size (11 weeks 2 days and 13 weeks 6 days).
4. Measure the amount of fluid in the skin at the back of the neck (nuchal translucency)
5. Check for any other obvious physical abnormalities of the fetus.
6. Check for uterine or ovarian abnormalities.
It is known that in all 12 week fetuses there is some fluid present in the skin at the back of the neck. Even in normal fetuses this layer can appear prominent. Multiple studies worldwide have shown that if there is more fluid than usual, the risk of Down syndrome is increased.
Combined screening test for Down syndrome
The best test currently available to screen for Down syndrome in pregnancy involves combining risks that are generated from a blood test as well as the ultrasound results and the maternal age.
This blood test of maternal serum screen is best taken at 10 weeks of pregnancy but can be used in the risk calculation if it is taken between 9 and 13 weeks. The blood test measures the levels of two pregnancy proteins called PAPP A and BhCG. In Down syndrome the PAPP A tends to be low and the BhCG tends to be high.
Using a computer program developed by researchers in London, you will be given a specific risk level of your fetus having Down syndrome. The risk assessment program combines the following measurements to calculate an overall or combined risk.
1. Age of the mother
2. Crown-Rump length of the fetus (age of the fetus)
3. Fetal Nuchal Translucency measurement
4. PAPP A
You will be given a personalized risk for your pregnancy being affected by Down syndrome. This will be in the form of a risk ratio.
E.g. 1 in 20 meaning 1 chance in 20
1 in 1000 meaning 1 chance in 1000
The high risk result is a risk greater than 1 in 200 (e.g. 1 in 120)
Most women will have a low risk (96%), however, 4% of women will be given a high risk. Most women with a high risk will still have a normal baby.
Women who are given a low risk may still have a baby with Down syndrome though this is very unlikely.
The scan does not tell you that the fetus had Down syndrome, nor down it tell you that it is normal. It is your decision what to do with the risk information. Individual couples can react quite differently to the same risk level. Some see a risk of 1 in 300 as being very high, while other might see the risk as being low.
As a rule of thumb, if the risk is more than 1 in 300 we would offer the option of having an invasive diagnostic test, either CVS (Chorion villus sampling) or an Amniocentesis. These tests can tell you with certainty whether or not the fetus has Down syndrome. Both these diagnostic needling techniques carry a small risk of miscarriage of about 1 in 200.
What other tests can be useful?
1. Second Trimester Maternal Serum Screening. This is a blood test taken from the pregnant woman between 14 to 20 weeks of pregnancy. This test measures 3 to 4 proteins in the woman’s blood that are only found in pregnancy. It is similar to the first trimester maternal serum screen used in combination with the 12 week scan. By combining the mother’s age with the level of these 4 proteins, it is possible to calculate a risk for her fetus having Down syndrome. This result is independent of an ultrasound scan, and can lead to a Down syndrome detection rate of about 70%. This test is offered at many public hospitals and is also very useful if the 12 week combined screen has been missed for any reason.
Note: It is usual for women to have only one of these blood tests in pregnancy, either the first trimester with the nuchal translucency scan, or the second trimester alone. There is only a single rebate associated with these tests.
20 week scan. All women are advised to have this scan regardless of their age, number of children or previous investigations (including CVS or Amniocentesis). At this scan a detailed anatomical assessment of your fetus will be made. If this is normal then may major fetal abnormalities can be excluded. This is considered the single most important scan of pregnancy. However, it is not a screening test for Down syndrome which is poorly detected at this later gestation. This is because the early skin thickening seen at 12 weeks disappears as the pregnancy progresses and has usually resolved by 20 weeks. This scan is best performed between 19 and 20 weeks of pregnancy.