Chorionic Villus Sampling Test
Chorionic villus sampling (CVS) is a test carried out during pregnancy to detect specific abnormalities in an unborn baby. A sample of cells is taken from the placenta (the organ that links the mother’s blood supply with her unborn baby’s) and tested for genetic defects.
CVS is offered in pregnancies where there is a high risk of the baby having a serious inherited condition. This could be because:
- you are over 35
- you have had a previous pregnancy where the baby had problems such as a chromosome abnormality or another serious health condition
- you have a family history of a condition such as cystic fibrosis or muscular dystrophy
- an antenatal screening test, such as the combined first trimester screening test or the non-invasive prenatal test (NIPT), has suggested that the baby may have a health problem
A CVS is usually done between 11 and 13 weeks of pregnancy. In special circumstances, the CVS might be carried out later, but it should not be done before 10 weeks of pregnancy.
The risk of CVS causing complications, such as miscarriage or birth defects in the baby, would be higher if it were carried out before week 10 of the pregnancy.
CVS or amniocentesis?
CVS is an alternative to amniocentesis, where a sample of the mother’s amniotic fluid is taken for testing. CVS can be carried out earlier than amniocentesis, which is usually carried out between weeks 15 and 20 of pregnancy.
Results from amniocentesis can take 2 to 3 weeks to come through. This may mean that your pregnancy is at a more advanced stage, around 20 weeks or more, before you can consider the results.
If you are at risk of passing a genetic condition on to your child, your doctor or midwife will be able to discuss the tests with you and explain why they might be necessary. They can also help you make a decision, if needed, about whether to continue with your pregnancy.
In some cases, you may be referred to a genetic counsellor (a healthcare professional trained in genetics). They will discuss your risk of passing on certain genetic conditions and can offer you advice about what to do when you get the results of CVS.
How is it performed?
During CVS, a sample of cells, called ‘chorionic villi cells’, is taken from your placenta using one of the procedures below:
- Transabdominal CVS — A needle is inserted through your abdomen, observed at all times by ultrasound. The needle does not enter the amniotic sac or go near the baby. The procedure is performed under local anaesthetic.
- Transcervical CVS — A tube is inserted through your cervix (the neck of the womb) and observed by ultrasound. You don’t need an anaesthetic — it’s similar to having a pap smear.
The test takes about 5 minutes, although the whole consultation will take about 30 minutes. CVS has been described as uncomfortable rather than painful, and there may be some cramps afterwards which are similar to menstrual cramps.
How does CVS work?
Early in pregnancy, the embryo divides into two parts. One part develops into the baby; the other part develops into the placenta.
The part of the embryo that forms the placenta starts out as finger-like sections known as ‘chorionic villi’. These burrow into the wall of the womb to get close to the mother’s blood vessels.
The chorionic villi are formed when the fertilised egg divides, which means they have exactly the same DNA as the embryo, including any possible genetic abnormality. Any defect in the chorionic villi will also be present in the fetus.
Risks of CVS
CVS increases your risk of miscarriage, but only 1 in every 100 pregnancies that end in miscarriage would be directly due to CVS.” This risk is in addition to the ‘background risk’ of miscarriage that all women have in early pregnancy due to natural causes. It is important to discuss your risk of miscarriage with your doctor, midwife or genetic counsellor.
The specific figure for the risk of a miscarriage depends on the experience of the doctor doing the test and the difficulty he or she has in obtaining the sample. It is therefore important that a CVS test is only carried out by a doctor experienced in this technique and it should be done after the 11th week of pregnancy.
You may have some vaginal bleeding after a CVS. Talk to your doctor about what you can expect after the testing and what symptoms to look out for.
Getting the results
After CVS has been carried out, the sample of chorionic villi will be taken to a laboratory so that the cells can be examined under a microscope. The number of chromosomes in the cells can be counted, and the structure of the chromosomes can be checked for any abnormalities. If the CVS is being carried out to test for a specific genetic disorder, the cells in the sample can also be tested for this. CVS is estimated to be about accurate in 99 cases out of 100.
However, it cannot test for every birth defect, and it may not give conclusive results. In about 1 of every 100 cases, the results of CVS cannot be completely certain that the chromosomes in the fetus are normal. If this happens, it may be necessary to have further tests, such as examining the chromosomes in cells from the parents. It may also be necessary to have amniocentesis (an alternative test in which a sample of amniotic fluid is taken from the mother) to confirm a diagnosis.
The first result should be available within a few days, and this will tell you whether a major chromosome problem has been discovered. The full results, including smaller, rarer conditions, can take 2 to 3 weeks to come back, although if the test is looking for a specific disorder the results may take up to a month. For most women who have CVS, the results of the procedure will be ‘normal’. This means the baby will not have any of the disorders that were tested for.
It is occasionally possible to have a normal result but then the baby is born either with the condition that was tested for or with another genetic condition. This is because a normal test result cannot exclude every possible genetic disorder. If your test is ‘positive’, your baby has the disorder that was being tested for. You will then be able to discuss the implications fully. There is no cure for most chromosomal conditions. Therefore, you need to consider your options carefully.
These will include:
- continuing with your pregnancy, while gathering information about the condition so that you are prepared for caring for your baby
- terminating the pregnancy
If you are considering termination, talk to your doctor or midwife. They can give you important information and advice and can refer you to a counsellor if you like.
Please note that Health2Wealth does not promote abortion.
Chorionic villus sampling (CVS) is a test carried out during pregnancy to detect specific abnormalities in an unborn baby.
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