CHORION VILLUS SAMPLING AND AMNIOCENTESIS
Chorion Villus Sampling and Amniocentesis
CVS (chorion villus sampling) and Amniocentesis are both diagnostic tests for chromosomal abnormalities of the unborn child. Both tests are equally accurate in the diagnosis of chromosomal abnormalities.
Who is offered an Amniocentesis or CVS?
1. Women of 35 years or over at the estimated of delivery.
2. Women in whom a screening test for Down syndrome has suggested a high risk for Down syndrome, or other similar chromosomal abnormality.
3. Women who have previously had a child with a chromosomal problem.
4. Women who are at low risk for chromosome abnormalities but are extremely anxious.
5. Women and their partners who are known to be at particular risk of carrying a baby with a disease such as cystic fibrosis, thalassemia, or the more rare metabolic disorders.
6. Women and their partners who are known to carry a chromosomal rearrangement that may lead to babies with more severe chromosomal problems.
What are chromosomal abnormalities, and what is Down syndrome?
Each cell in the body normally contains 23 pairs of chromosomes. Half the chromosomes come from each parent, and pair up to make 46 chromosomes. Normal males are denotes as 46 XY and normal females as 46 XX. Sometimes an error occurs during fertilization and the fetus may end up with more or less chromosomes.
Down syndrome refers to an extra chromosome 21 and this is denoted as 47 +21. Another name commonly used for Down syndrome is Trisomy 21 (meaning 3 chromosomes of number 21). Babies, if born alive with Down syndrome, usually can expect to have a healthy long life. The main problem these children face is one of intellectual delay and learning difficulties. Most will need a sheltered environment throughout their lives. Some babies with Down syndrome have major structural abnormalities, such as heart abnormalities, though the majority born alive will not have major problems.
1. Trisomy 18 – Edwards syndrome (usually lethal)
2. Trisomy 13 – Patau syndrome (usually lethal)
3. XO – Turners syndrome (infertile females)
4. XXY – Kleinfelters syndrome (infertile males)
Why are older women offered an Amniocentesis or CVS?
As the age of a pregnant woman increases, the chance of her having a fetus with Down syndrome or other chromosomal abnormality increases. This is because women store their eggs from the time that they are babies, and as the years go by more errors in cell division start to occur. These errors are part of the normal ageing process, and are not influenced by the general health of the mother, or how many healthy children she has.
What is the difference between CVS and Amniocentesis?
These two tests take a different tissue sample from the pregnancy that can then be tested from chromosomal abnormalities. Both tests are equally effective at diagnosing Down syndrome. This list below compares the main features of each test.
| TEST | CVS | AMNIOCENTESIS |
| Timing of Test | 10-14 weeks | 15-20 weeks |
| Tissue Sample | Placental tissue biopsy | Amniotic fluid (fetal urine) |
| Time First Used | 1980’s | 1960’s |
| Miscarriage Rate | 1 in 200 | Less than 1 in 200 |
| Background miscarriage rate at the time of the test | 1 in 50 | 1 in 100 |
| Concurrent tests | Down syndrome screening 11-13 weeks | Anatomy scan after 17 weeks AFP may suggest a high risk for spina bifida and other defects |
| Main advantages | Early diagnosis
Best tissue for DNA analysis and other rare abnormalities |
Slightly lower miscarriage rate (minimal difference) |
| Rare complications | Transverse Limb defects when the CVS is performed before 9 weeks | Premature rupture of the membranes
|
Why is an ultrasound examination needed with these tests?
Before the test is performed an ultrasound scan will be performed to check the following:-
1. That the fetal heart beat is clearly seen.
2. To localize the placenta and fetus.
3. To check the amount of fluid around the baby before Amniocentesis.
4. To assess if twins are present.
5. To check whether the fetus looks normal. If the fetus has a suspected problem a rapid result may be organized (see FISH).
During the test the ultrasound will be used to guide the needle to the best and safest position, thus minimizing the potential for damage to the pregnancy and other organs.
After the test a scan will be performed to check the fetus is still moving and has a heart beat.
Safety Issues
All our procedures are performed using a needle guide. This means that the needle has a fixed route into the uterus. This has been shown to be the safest way of performing these tests, and minimizes any damage to other organs.
CVS and the Amniocentesis are both performed by passing the needle through the mother’s abdominal wall. A sterile technique is used thus minimizing infection and pregnancy loss from the procedure. Trans-vaginal CVS is no longer performed in our practice because of the higher miscarriage rate associated with this route.
What will happen on the day of the test?
The doctor who will perform the test will also perform your scan. At this time the procedure will be explained in detail, and any questions answered.
The procedure is performed in the Ultrasound room under ultrasound guidance. This amount of discomfort with both these tests is small. Most women find that these tests are like having blood taken and are easily tolerable. The whole procedure takes only a few minutes. The needle is in the mother’s abdomen for only a minute or two. At the end of the procedure the fetus is checked.
A full bladder is not required for either the CVS or the Amniocentesis. A little urine in the bladder may be useful in some circumstances. If you feel uncomfortably full, please ask our staff if you may go to the toilet.
All women undergoing these tests are asked to wait 30 minutes before heading home to rest. Occasionally a woman may faint after a test, and if you are prone to this you should tell the staff so that arrangements can be made for you to lie down after the procedure.
We recommend that a companion drives you home just in case you feel a little pain or faintness, however most women are quite capable of driving themselves home if need be. We recommend that patients rest at home until the following day, although there is no reason to go to bed.
Arrangements should be made for another adult to care for children under the age of 5 for the rest of the day. Very active sports or very physical jobs should be avoided for a day or two, though other normal activities are fine.
What complications can occur?
1. Miscarriage. CVS and Amniocentesis both have a small risk of causing a miscarriage. For both tests this risk is less that the background risk of miscarriage at the same gestation. The world literature suggests that amniocentesis has a slightly lower miscarriage rate than CVS. In our hands the risk of miscarriage from a CVS is well below that suggested by the world literature. This is probably related to our experienced doctors, and the fact that we use a needle guide and no longer perform trans-vaginal CVS (see Safety Issues). If a miscarriage occurs due to an Amniocentesis or a CVS it usually happens within 2 weeks of the procedure.
2. Bleeding. This complication is more common after CVS than Amniocentesis. It is still a rare complication. If it occurs we advise that you contact your doctor immediately. We will organize to scan the pregnancy again to make sure that the fetus is still alive. Most women who have bleeding will not miscarry, but will go on to have a normal healthy baby.
3. Fluid Leakage. This is a rare complication of Amniocentesis. It occurs in approximately 1 in 100 procedures. In most women the leakage will stop spontaneously, but in a small proportion it may lead to ongoing loss and miscarriage. If you suspect any fluid loss you should contact your doctor so that it can be confirmed or excluded. Another scan will be ordered if there is a high suspicion of this complication. Fluid leakage is very rare after CVS.
4. Fetal risk. With CVS after 10 weeks and Amniocentesis after 15 weeks there are no reported fetal abnormalities due to the tests. Some fetal abnormalities will be discovered at a later date, which are not due to the test, but would have occurred anyway. Because of this, all women are offered a scan at 18 to 20 weeks to try and detect this sort of problem. CVS performed before 9 weeks is thought to be associated with rare limb deformities. We have not had a case like this in our practice taking into account the natural background rate for these abnormalities.
5. Laboratory failure. Very rarely there can be a problem in the laboratory, which means that the specimen fails to give a result. In a very small percent of specimens the result may be difficult to interpret, and it may be necessary to repeat a test.
Results:
The laboratory testing is very similar for both Amniocentesis and CVS. The laboratory process is complex and involves and involves growing cells until there are enough of them in the middle of the dividing phase to allow the chromosomes to be examined. This process takes between 10 and 14 days. Your doctor will receive the results as soon as it is available and will notify you of the result.
Rapid results: A new test has been devised to give a quicker result. This test is called a FISH test (Fluorescent In Situ Hybridisation) and it is designed to test for Trisomy 21 (Down syndrome) and several other common abnormalities. It is not considered a complete test and in all circumstances the full result will be available in 10 to 14 days. This test is offered to couples that are very anxious, or at a particularly high risk of abnormalities. This test has no Medicare Rebate and costs an additional $150-$200. If you wish to have this additional test please let us know.
