Early in pregnancy, the developing embryo splits into two parts: one part will eventually develop into the baby, and the other into the placenta. This vital organ supplies the baby with blood, oxygen and nutrients and removes waste for the duration of pregnancy.
The part of the embryo that will become the placenta initially takes the form of small finger-shaped structures called chorionic villi. These structures probe the wall of the uterus to make contact with the mother's blood supply. Because the villi originated from the same fertilised egg as the foetus, this means they are both made up of the same genetic code (DNA) and therefore any genetic abnormalities that will be present in the baby will also be present in the villi.
Chorionic villus sampling (CVS) involves taking a small selection of chorionic villus cells for genetic testing. This can reveal the presence of abnormalities that cause Down’s syndrome, cystic fibrosis, sickle cell disease, thalassaemia and other genetic (or inherited) disorders.
This article on chorionic villus sampling is by Kathryn Senior, a freelance journalist who writes health, medical, biological, and pharmaceutical articles for national and international journals, newsletters and web sites.
Why is chorionic villus sampling performed?
CVS is not offered routinely to all women due to the risks involved. You may have chorionic villus sampling:
- If you are older than 35
- If you have a past history or family of genetic or chromosomal abnormalities
- If you have had a previous test such as a nuchal fold scan that suggests have a possible genetic disorder
When and how is it performed?
Chorionic villus sampling is usually performed between 10 and 13 weeks of pregnancy. This is usually a transcervical/transvaginal CVS, and a transabdominal CVS may be performed from 13 weeks onwards.
You will need a full bladder for chorionic villus sampling and the main steps involved are:
- An ultrasound scan will tell you how many weeks pregnant you are and will determine the position of the placenta. Once that’s done, the doctor will be able to decide whether it’s best for you to have a transvaginal or transabdominal CVS.
- Ultrasound will be used as a guide throughout and to check the baby afterwards:
- During transvaginal CVS, your doctor inserts a pair of fine forceps or a small tube into your vagina and through your cervix to the placenta.
- During transabdominal CVS, a needle is pushed through your abdomen and inserted into your uterus to reach the placenta. You may be given a local anaesthetic for this method of chorionic villus sampling to reduce discomfort.
- A small sample of cells is then taken from the placenta and removed for testing.
- The CVS test takes around 20 minutes to perform in total, and results are usually available within 1 to 2 weeks, but can take up to 3 weeks if the test needs to be sent away.
Dealing with the results
The test results from a CVS will show if your baby has a known genetic or inherited abnormality. The DNA analysis of the cells taken from the chorionic villus will show if your child will be born to face disability or a lower life expectancy. You will then be given plenty of support to decide what to do. This is a very personal decision and full of anguish for both parents and their families. You will be given counselling and advice but the choice of whether to keep your child, or whether to have a planned termination will, ultimately, depend on how you and your partner feel.
What are the advantages of chorionic villus sampling compared with amniocentesis?
CVS can be performed earlier in pregnancy than amniocentesis, which is generally only performed in the second trimester. It therefore offers an earlier opportunity to make choices if a foetal abnormality is detected.
However, it is important to remember that, unlike amniocentesis, CVS does not test for structural problems such as neural tube defects. Therefore, it is important for you to have blood tests for serum alpha-fetoprotein and other diagnostic tests if screening for neural tube defects is considered necessary.
Are there any risks involved?
Approximately 0.5–2% of women miscarry as a result of CVS. The CVS procedure can sometimes cause bleeding or infection. In particular, the transvaginal method can sometimes lead to cervical polyps or fibroids, or problems with the position of the placenta. There are risks of limb deformities when CVS is performed earlier than 10 weeks into pregnancy, and so the procedure is generally not carried until 10 weeks or later.
Women with HIV are at increased risk of passing the virus to their baby if a CVS is performed, and so the procedure is best avoided if you are HIV positive.
Is this right for me?
Having a diagnostic test can be emotionally and physically demanding. It is important to discuss with your doctor the disorders that CVS can test for, how it will be carried out, the risks, and possible outcomes, as well as the choices you will have afterwards. Your doctor will take all the medical information available into account when advising you whether the CVS test is suitable but the final decision is yours.