CVS and Amniocentesis
These are prenatal diagnostic tests. They are definitive “yes/ no” tests
(rather than screening tests, which only give likelihood).
CVS and Amniocentesis
They are most commonly offered to check the chromosomes of the baby for conditions such as Down Syndrome, They are also performed for other specific reasons, such as DNA tests. We will need your blood group in writing (see After the Test below).
CVS (Chorionic Villus Sampling) is the early test. A small sample of the developing placenta (the fine placental fronds are called the chorionic villi, at this early stage) are taken for analysis.
It is performed around 11 weeks of pregnancy for DNA tests or pre-arranged CVS, or, 12 weeks if following a high-risk first trimester screen. CVS is the preferred procedure for DNA tests.
Amniocentesis is the ‘gold standard’ chromosome testing. Some fluid is taken from the gestational sac for testing at, or after, 15 weeks. The skin cells of the baby, which are floating in the amniotic fluid, are grown for analysis. Therefore, the result is direct from the fetus. It also carries the least risk of any diagnostic test.
Some common reasons for amniocentesis or CVS
- Advanced maternal age: It has long been known that the risk of having a baby with Down Syndrome (Trisomy 21) gets higher as the mother gets older. Testing for age alone has become less common for women in their thirties. This is since other screening tests are now used in combination with age.
Mother’s age risk of a live born baby with Down Syndrome:
37 years 1 in 240
38 years 1 in 190
40 years 1 in 100
45 years 1 in 30
The rate of Down Syndrome is higher earlier in pregnancy. This is due to the fact that the miscarriage rate is higher in Down’s fetuses, compared to those with normal chromosomes.
- Screening tests (such as nuchal translucency screening, blood tests, or a combination of these, with maternal age): these indicate an increased risk of a chromosomal disorder. First trimester screening at 12 weeks is the commonest of these tests in WA.
- Where a previous baby had a chromosome problem: definitive tests can be particularly helpful for women with a previous abnormal outcome.
- Ultrasound findings suggest an increased risk: This relates to an increase risk of a chromosome problem.
- Occasionally Amniocentesis is performed for anxiety: This is when the medical reasons are not clear cut. However, it is important to understand that an Amniocentesis checks only for chromosome disorders and is not a guarantee against other problems.
- DNA tests: these cover some serious disorders, such as cystic fibrosis or muscular dystrophy. These are individual gene tests and are tested when there is a known risk of the specific disorder. Families may need to have tests before the pregnancy. CVS is the procedure of choice for DNA tests, but DNA tests are not part of a “routine” CVS.
- CVS does not test for spina bifida: although many years ago Amniocentesis was used for a spina bifida diagnosis, this is now diagnosed by ultrasound. Spina bifida is not more common with increasing age.
Procedure for the tests
Before either test an ultrasound examination is performed to check the fetus, confirm the progress for the stage of pregnancy and locate the placenta.
Transabdominal CVS (through the abdominal wall): this is the common route of CVS. The skin is washed with antiseptic, then some local anaesthetic is given. A needle is inserted through the skin into the placenta, under ultrasound guidance. The sample of the placenta is drawn up through the needle with gentle suction. Occasionally, a second pass is required if the sample is not sufficient for the particular tests required.
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Transcervical CVS (through the neck of the womb): this is now rarely used. It is much like having a Pap smear done, although it takes slightly longer. It is useful when a combination of conditions which make the transabdominal approach impossible or difficult. It is not applicable after 11 ½ weeks.
Amniocentesis: the skin is cleansed with antiseptic. A fine needle is then passed under ultrasound vision through the skin and down into the fluid which surrounds the baby. This is so that a sample of the fluid can be taken. On average, it is no more painful than a blood test, although the abdominal wall may feel sore afterwards.
After the test
We will ask you to sit quietly for several minutes after the test, and to take things quietly at home for at least a day. After having a needle through your abdominal wall, you can expect to feel sore for a day or two. If in doubt or if you are worried about any symptoms, contact your Doctor.
One in five women have a Rhesus negative blood group. If you are one of these, then you will require an “Anti-D” injection to prevent the complication of Rhesus Disease (where the mother makes antibodies for the fetus).
Getting the results
A rapid result for chromosome tests is usually available within 48 hours, with a charge from the laboratory. This preliminary test is for Down Syndrome and other common major chromosome abnormalities. The final results require culture of the specimen. This can take around 2 weeks and will be sent to your referring doctor. The chromosomes contain information of the sex of the baby, so you should consider whether you wish to know this.
What are the risks of CVS and amniocentesis?
The main risk is miscarriage. This is around 1% above the background rate for CVS and 0.5% (1 in 200) for Amniocentesis (when performed by an experienced ultrasound Doctor).
As with any test, there are rare circumstances where a result is not available. Such as, failure of cells to grow in the laboratory or where a particular CVS result is not considered trustworthy. This occurs in less than 1% of CVS, where a confirmatory Amniocentesis is then advisable. Therefore, it is important that you make sure your Doctor knows how to contact you at all times after the test.
There are no other significant risks in tests performed after 10 weeks.
Amniocentesis is very close to 100% accurate for the chromosome result. However, even with both ultrasound and Amniocentesis, there is no guarantee that the baby will be born without any birth defect or mental retardation.
What if the result is abnormal?
You would be told what the abnormality was and how this could affect your baby. You could use this information to consider whether or not you wanted to continue the pregnancy.
There would be a full chance to discuss the situation according to the results and your wishes.
Is it better to have an amniocentesis or CVS?
This is a very individual question and depends on your preference, level of risk, level of anxiety and the timing of any information leading to testing. The advantages of CVS are in the early timing and chance of a faster result. This allows:
1. Privacy before the pregnancy is apparent.
2. No delay between recognising a high-risk situation and diagnostic testing.
3. The reassurance of getting a normal result early.
4. A safer medical procedure to terminate the pregnancy if the result is abnormal.
These factors need to be balanced against the slightly higher risk of miscarriage compared to Amniocentesis (1% for CVS compared to ½ % for amniocentesis).
For more information on CVS and Amniocentesis, download the pdf documents below.