Chorionic villus sampling (CVS) is the technique for collecting a small sample of the developing placenta for chromosome testing. The fetus and placenta both develop from the fertilised egg and therefore chromosome tests on the placenta almost always give the same result as tests on the fetus itself.

CVS is used to test for Down Syndrome or other chromosome abnormalities in the fetuses of couples who are at increased risk of having an affected child. These include women identified by screening tests, women aged 35 years or more, or those who have had a baby with Down Syndrome, or who have an inherited chromosome problem in the family. It can also be used to diagnose a large number of inherited disorders such as cystic fibrosis, muscular dystrophy, thalassaemia and fragile-X mental retardation, in couples known to be at increased risk of having an affected child. There are other less common reasons for CVS as well. Spina bifida and other neural tube defects cannot be diagnosed by CVS. CVS is an out-patient procedure usually performed between 10 and 12 weeks of pregnancy. Partners are welcome to attend. It takes about 30 minutes and no special preparation is needed. Most of this time is spent being scanned and preparing equipment; it takes only a few minutes to obtain the sample of placenta. Your blood group needs to be known and if it is rhesus negative you will be given an injection after the procedure. Your Consultant will explain why this is necessary.

The results of CVS are almost always accurate (99 in 100 tests). In two in one hundred tests (2%) there is difference between the CVS result and the result that would have been obtained if the fetus had been tested directly. In other words, the chromosomes of the fetus and its placenta can be different. The laboratory can usually tell when this has happened because of the type of result obtained. Most often the fetus has normal chromosomes and an amniocentesis at fifteen+ weeks will be recommended to confirm this. Rarely, perhaps one in one thousand tests (0.1%), the placenta has normal chromosomes when the fetus has a serious chromosome problem and a pregnancy will result in the birth of an abnormal child. This type of error cannot be prevented but the chance of it happening is very small.

CVS is no longer carried out before 11 weeks of pregnancy because of the small risk of the test causing an abnormality of the hands, feet and face of the developing fetus. This will be discussed fully by the Consultant carrying out the test.

The piece of placenta is sent to the CGS laboratory in London for testing. The test that will be done most often is a chromosome test. This looks for Down Syndrome and most other chromosome problems. Provisional results should be available in 72 hours and a confirmed result in 10-14 days. Most women will be 12-13 weeks pregnant when they receive their final results. The sex of the fetus will be revealed by the chromosome tests and the parents can choose whether or not they wish to know it. CVS cannot be requested simply to discover the sex of the fetus with the intention of terminating the pregnancy if the fetus is not the right one.

CVS is a relatively safe procedure but there is a small risk of miscarriage associated with it. One in one hundred (1%) women will miscarry as a result of CVS. If CVS shows the fetus to have an abnormality you will be told what the problem means and what options are available. You and your partner can then decide whether or not you wish to continue the pregnancy. Couples considering CVS should decide early on whether or not they could accept termination of pregnancy. If they do not, all that testing can provide is advanced knowledge that the baby does or does not have a specific abnormality.

Patient procedure notes:

• The Consultant carrying out the test will perform an ultrasound scan to confirm the age of pregnancy and the site of the placenta. 

• At the beginning of the procedure a site will be selected on the abdomen through which the sampling needle will be inserted. A patch of skin on the abdomen will be cleaned with antiseptic and some local anaesthetic injected.

• A fine needle will be guided through the abdomen into the developing placenta, its position checked by ultrasound examination and a syringe attached to the top of the needle. You may be able to watch what is happening on the video screen. 

• A small piece of the placenta is removed by sucking hard with a small syringe, the needle is then withdrawn. You will be asked to rest at home for the remainder of the day and to exclude vigorous activity for two more days.