Chorionic villus sampling
What is chorionic villus sampling?
Chorionic villus sampling is a test which involves obtaining a small sample of tissue, the chorionic villi from the developing placenta.
The placenta usually has the same makeup as the baby. It can be tested for chromosomal or DNA abnormalities, like Down syndrome.
Who may be offered CVS?
Most testing by CVS is offered to patients who are at particular risk of chromosomal abnormality such as Down syndrome.
This could include:
- Women with a high risk cell free fetal DNA (NIPT) result
- Women with a high risk nuchal translucency result
- Women over the age of 35
- Women who have had a previous affected baby or pregnancy
- Occasionally it is performed for those who are particularly anxious about the risk of chromosomal abnormality
How is the test performed?
The skin of the lower abdominal wall is cleansed with an antiseptic alcohol based solution.
The skin and underlying tissues are injected with local anaesthetic.
The discomfort felt during the injection of the local anaesthetic is similar to that felt when blood is drawn from the forearm.
With ultrasound control a fine needle is then guided into the placenta and a biopsy of placenta tissue is taken (chorionic villi).
Sometimes a dragging sensation is felt in the pelvis or in the legs during the procedure. This is to be expected.
Also sometimes more than one biopsy is required.
If the placenta is developing on the posterior uterine wall and low in the uterus it may be easier to perform the procedure trans-cervically.A speculum is inserted into the vagina and the cervix is cleansed with an
A special plastic catheter is guided into the future placenta with the help of ultrasound. Placental tissue is gently aspirated.
This procedure is usually completely painless.
When is the test performed?
Ideally the test is performed between 11-12 weeks
gestation. However technically it can be performed as early as 10
weeks and as late as 40 weeks.
What preparation do I need before the test?
A moderately full bladder is preferable.
This brings the pregnancy up into the abdominal cavity and thus accessible for the needle test.
If you begin to feel very uncomfortable with a distended bladder then try to let some urine out as this may also hinder the test.
A scan is performed before the test to:
- Make sure the baby is alive and well
- Determine whether there are twins or more
- Check for major abnormalities in the baby
- Locate the placenta
Finally it is also very important to know what your blood group is before the test ie Rh positive or negative.
If possible ask your doctor to provide this information or bring along you blood group card at the time of the test.
If you are a negative blood group you will need an injection of anti-D after the test.
This will be explained further by the Doctor at the time of the test.
What happens to the sample after it has been taken?
The specimen is sent to the laboratory and processed. The tissue is placed in a culture medium and then into an incubator for several days.
When there are sufficient number of dividing cells the specimen is removed from the incubator and the placental cells are split open with an enzyme. The individual chromosomes are counted and analysed.
Every cell should contain 23 pairs of chromosomes. The
test looks for an extra chromosome number 21 (Down
Each of the other 22 pairs of chromosomes are also examined. This includes chromosome pairs number 1,2,3,4, and so on up to the sex chromosomes X & Y (pair no 23).
Therefore this test not only excludes an extra chromosome number 21 (Down syndrome) but it excludes extra chromosomes in the other 22 chromosome pairs.
Each individual chromosome pair is then stained with a special dye and examined under ultraviolet light.
The individual bands of each chromosome are examined in great detail for subtle genetic abnormalities such as:
- Insertion of genetic material into a chromosome
- Deletion of genetic material from a chromosome
- Exchanging of genetic material between chromosomes (called translocations).
So this test excludes not only Down syndrome but a wide variety of additional subtle and major chromosome abnormalities.
What should I do after the test?
It is recommended that you rest for the remainder of the day. Many women feel tired and emotional and a little bit sore in the tummy.
This does not mean you should confine yourself to bed. If you feel well enough to go back to work this is fine, but avoid any strenuous activity like lifting heavy weights.
Most patients experience a short duration of mild crampy period-like pains, lower abdominal pains or even leg pains after the test. This is most likely to occur after the local anaesthetic wears off, ie within the first half hour after the test.
Some patients experience slight vaginal blood spotting after the test which is also not unusual.
What are the risks of the test?
There is less than 0.5% risk of miscarriage with the test.
There is also a pre-existing background risk of miscarriage which is approximately 2% at this gestation.
To minimize the risk of infection, strict sterile technique is followed.
The needle itself stays outside the gestation sac, so the fetus is never touched. The sample is taken from the chorionic villi outside the gestation sac.
After the test
We ask you to stay for 10-15 minutes to recover and have a cup of tea. We will give you a list of things to look out for after the procedure. A small amount of spotting is quite common as we have taken a little bit of placenta and it may bleed a little until itself.
But warning signs of miscarriage include strong period like pains with fresh red bleeding. Call us or your doctor if you are concerned.
If a miscarriage does occur due to the procedure, it is usually in the first 24-48 hrs after the test.
If you are worried, you are welcome to come in for an ultrasound to check the fetal heart motion.
The laboratory tests
Using special procedures in the laboratory, cells are grown in a culture medium. The cells are then fixed and stained. With high power microscopy the chromosomes in these cells can be examined and in this way it possible to find out if the baby has Down syndrome, or any of the other known less common chromosome problems. This is called a karyotype. Also, by looking at the chromosomes, the sex of the baby can be determined.
In order to get faster results, the laboratories have developed a technique called fast Fluorescent In-Situ Hybridization, where some of the cells from the sample are stained with a fluorescent dye. This can give us results for chromosomes 13, 18, 21, X & Y, in 24 hours. This is especially popular for women who have had a high risk result for Down syndrome (Trisomy 21.)
Chromosome microarray (CMA) testing
Chromosome microarray (CMA) testing is a detailed genetic test which can detect extra or missing segments of DNA. As there are usually two copies of each chromosome, there should be two copies of each segment of DNA. Using the chorionic villus sample the microarray can look for a variation in the number of copies of the DNA. This is known as a copy number variant. If the test finds a copy number variant, the laboratory will check which genes are duplicated or missing. From this information we can predict what sort of problems the baby will have.
What happens with the results?
We ring you with the results as soon as they become available and we also forward them to your doctor.
Fast FISH results are usually available in 24-48 hours. The full karyotype takes 10-14 days; microarrays take 5-10 days. We ring you with all of your results whether they are normal or not. If the results are not normal we will suggest that you come in and see one of the specialists, geneticists or genetic counselors. We can help you to understand what the results mean for the pregnancy and whether other tests are recommended.
Issues with test results
Occasionally there are problems with the sample. It may fail to grow, or it may give some sort of confusing result.
These problems are very uncommon. They occur in <1% of all CVS tests we do.
If they do occur we will contact you and discuss what you should do next.
Sometimes we have to test you and your partner to see if one of you has a similar chromosomal pattern. If further fetal testing is needed, you still have the choice of an amniocentesis test at 15-16 week of pregnancy to clarify the situation.