In recent years, several new tests have become available primarily aimed at testing for Down syndrome. These tests are optional tests in addition to the usual blood and detailed ultrasound examination at 18 to 20 weeks. This information sheet provides a summary of the tests available and is intended to be a reminder of the issues that we discussed today.
What is Down Syndrome?
Down syndrome (Trisomy 21) is a condition in which an extra chromosome 21 is carried in each of the cells of the body. The extra chromosome 21 results in intellectual disability of varying degrees and may cause problems with the heart, kidneys, bowel, eyesight or hearing. Although the chance of having a baby with the Down syndrome increases with mother’s age, Down syndrome babies may also be born to younger mothers. The following table indicates the risk of having a baby born with Down syndrome.
Maternal Age at the expected time of delivery Approximate risk of baby being born with Down’s Syndrome
25 1 in 1250
30 1 in 890
35 1 in 355
37 1 in 220
40 1 in 90
45 1 in 23
What is Edward Syndrome?
Edward syndrome (Trisomy 18) is a very serious condition in which an extra chromosome 18 is carried in each of the cells of the body. This condition occurs much less often than Down syndrome. Most pregnancies affected by Edward syndrome will miscarry. Those that continue may have a baby affected with heart, kidney or lung problems. Many other features may be apparent on ultrasound examination.
What are neural tube defects?
The most common neural tube defects are spina bifida and anencephaly. Babies with spina bifida have an opening in the bones of the spine which may result in damage to the nerves controlling the lower part of the body. This can cause weakness and paralysis of the legs and sometimes inability to control the bowel and the bladder. In anencephaly, the brain does not develop properly and the baby will not survive. Around 1:750 pregnancies are affected by a neural tube defect if the mother has not been taking folate around the time of conception. This risk is dramatically reduced, although not eliminated, by taking folate.
How reliable are these tests?
The reliability of a test depends on its ability to detect a problem if it is there: the sensitivity of the test. Sensitivity of 80% means that a test will detect 80% of babies who have the problem (miss 20%). The other important factor in assessing the reliability of the test is the false positive rate. A test is considered to give a false positive result if it suggests there is a problem but the subsequently shown not to be there at all. Ideally a test would have 100% sensitivity and 0% false positive rate but in practice this does not happen.
Low risk means that the risk of having a baby with Down syndrome, Edward syndrome or a neural tube defect is low. Increased risk means that there is an above average risk of having a baby with one of these conditions but it does not necessarily mean that there is a problem with your unborn baby. It means that further tests should be considered to determine if there is a problem. It is important to remember that most women with an increased risk will go on to have a normal baby.
OPTIONS FOR PRENATAL TESTING
1. No testing.
With the risk of these conditions being very low, it may well be your preference not to have any testing at all. This is a completely appropriate decision that is entirely yours to make.
2. Nuchal translucency (NT) scan
Between 11-14 weeks all babies have a small amount of fluid under the skin at the back of the neck. An excessive amount of fluid in this area is associated with a higher chance of having a baby with Down syndrome or other abnormalities such as heart or kidney problems. This can be measured by ultrasound and is called nuchal translucency. If there is an excessive amount of fluid then further investigations will be recommended. An ultrasound at this stage also has additional benefits such as accurate dating the pregnancy, diagnosis of multiple pregnancies and other unrelated abnormalities.
3. Combined first trimester test
The combined first trimester test for Down syndrome involves combining the results of the nuchal translucency scan with the results of a blood test performed at least 2 to 4 days prior to the ultrasound.
4. Maternal serum screening (MSS)
Maternal serum screening is a blood test between 14-20 week for pregnant women to determine if they may be at risk of having a baby with Down syndrome, Edward syndrome or a neural tube defect.
5. Chorionic villus sampling (CVS)
Chorionic villus sampling involves passing a needle into the placenta between 10 and 13 weeks gestation. A small amount of placental tissue is taken for chromosome analysis. Results take approximately seven working days to become available. Some ultrasound centres will offer you the option of FISH testing. This is a very rapid test for Down syndrome with the result being available the following day at an additional cost. Further details of the actual procedure will be given to you if this is the option you choose. The risk of miscarriage from the procedure itself is 0.5-1.0%. You must remember that there is a risk of spontaneous miscarriage at this gestation of approximately 2%. The total risk is therefore 2.5-3.0% although only a small proportion of this risk of miscarriage is actually as a result of the CVS test.
6. Amniocentesis
Amniocentesis involves taking a sample of amniotic fluid from around the developing baby at approximately 16 weeks gestation. Only a small amount of fluid is taken but this is enough for a full chromosome analysis to be performed. Results take approximately seven working days to become available. Some ultrasound centres will offer you the option of FISH testing. This is a very rapid test for Down syndrome with the result being available the following day at an additional cost. The risk of miscarriage from the procedure itself is approximately 0.5%. The risk of spontaneous miscarriage at this gestation is 0.5%. The total risk from the procedure and spontaneous miscarriage is 1.0%.
Summary
You should not feel pressured to undertake any of these tests if you do not wish to do so. It is important to recognise that the choice of testing is yours. I will give you advice on the different types of tests available but the ultimate decision must rest with you. The following table summaries the sensitivity, false positive rates and risk of each test for Down's syndrome
| Test | Weeks of test | Detection rate | False positive rate | Risk of miscarriage from the test |
| No test | 0% | 0% | 0% | |
| NT | 11-14 | 80% | 5% | 0% |
| Combined 1st trimester test | 11-14 | 90% | 5% | 0% |
| 2nd trimester MSS | 14-20 | 80% | 5% | 0% |
| CVS | 10-13 | 100% | 0% | 0.5-1% 评论 谢谢LZ分享！等我明天有空帮LZ翻译一下，就方便很多英文不好的姐妹们。 评论 呀，花生还没有翻译捏？都好多个明天了捏。。。 评论 还没有翻译吗？ 评论 自己上中文网站上看看就行了。。 别浪费别人的时间翻译了。。 评论 GP那里有中文版的，不用翻译啦，反正按年龄看看概率多少就行。 澳洲中文论坛热点
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