Chorionic villus sampling (CVS)

If you are considering chorionic villus sampling (CVS) or have an operation planned, it is important to know all you can about it. This includes:

• why you need this operation

• what it will be like

• how it will affect you

• what risks are involved

• any alternatives.

The information here is a guide to common medical practice. Each hospital and doctor will have slightly different ways of doing things, so you should follow their guidance where it is different from the information given here. Because all patients, conditions and treatments vary it cannot cover everything. Use this information when making your treatment choices with your doctors. You should mention any worries you have. Remember that you can ask for more information at any time.

What is a CVS?

CVS is short for chorionic villus sampling. It is a test to check for abnormal chromosomes in your baby. We can also use a CVS to check the baby for a number of other inherited conditions, such as sickle-cell disease.

During the test we remove a small sample from your placenta. The test is guided by ultrasound. We send the sample to the laboratory for examination. We can do this test from as early as 11 weeks gestation. It is not advised before 11 weeks. Results are ready two weeks later.

There are two different methods of doing the procedure. The method used depends on the position of your placenta and the preference of the doctor doing the procedure. In some cases we put a needle through your abdomen to take a sample. This is called a transabdominal CVS. Alternatively, we do the procedure through your vagina and cervix. This is called a transcervical CVS.

What are chromosomes?

Chromosomes are tiny structures that lie within each cell of the body. There are 46 chromosomes in a human cell. We get 23 from our mother and 23 from our father, arranged as 23 pairs. They determine whether you will be male or female, how you will look and, more importantly, how your body works. Abnormal chromosomes may cause conditions such as Down’s syndrome.

What is Down’s syndrome?

This condition is caused by an extra chromosome in pair 21 and is also called Trisomy 21. People with Down’s syndrome have low intelligence, slanting eyes and a flat face. They have a single crease in the palm of their hands, called a simian crease, and a widened gap between their first and second toes, called a sandal gap. More importantly, they can have serious heart defects and abnormalities of their bowels and kidneys. They are also prone to other diseases, such as leukaemia, and mental disorders.

However, people with Down’s syndrome can also have a normal life span and some children are more affected than others.

Why am I having a CVS?

There are many reasons for doing a CVS. These include:

• Abnormality seen on ultrasound scan - Pregnant women usually have an ultrasound scan at 12 weeks gestation. This is called a nuchal translucency scan. During the scan we look at the developing baby in your womb and check for an increase in fluid behind the baby’s neck. If an increase is seen, this gives you a higher chance of having a baby with Down’s syndrome. If the ultrasound gives any cause for concern, a CVS can confirm whether the baby has Down’s syndrome.
• Age of the mother - All women have a risk of having a baby with Down’s syndrome but the risk increases with age. Older mothers have a higher risk so we may offer you a CVS if you are over 37 years of age.
• Previous abnormalities - If you have had a previous pregnancy affected by abnormal chromosomes we may offer you a CVS.
• Screening for diseases - We can diagnose many diseases using a CVS. Examples include cystic fibrosis when both parents are cystic fibrosis carriers, or sickle-cell disease from two sickle-cell trait parents.

If you are offered a CVS your obstetrician will counsel you on the possible outcomes. S/he will discuss the various options, including not doing a test, doing a test taking the risks into consideration and what to do in the event of a positive result, including a termination of the pregnancy.

The aim

The aim of a CVS is to check your unborn baby for any abnormal chromosomes or inherited conditions. The test is 98% accurate.

The benefits

The test will confirm whether your baby has an abnormality or not. A normal result will put you and your partner’s minds at rest. An abnormal result will tell you if your baby has Down’s syndrome or some other inherited condition. This would enable you to make a decision to continue with the pregnancy or not. The test results are usually received during early pregnancy so if you decide to terminate the pregnancy it can be done at an early stage.

Are there any alternatives?

An amniocentesis is another test for abnormal chromosomes. In this test we take a sample of the amniotic fluid surrounding your developing baby. This test can be done after 16 weeks gestation. If an abnormality is found on your 12-week ultrasound scan it will be too early for an amniocentesis. A CVS is usually done between 11 and 16 weeks gestation rather than an amniocentesis.

What if you do nothing?  

If you do not have the test you will not know if your baby has an abnormality. If you are in a high-risk group we advise having the test.

Author: Dr. Chineze Otigbah MRCOG. Consultant obstetrician and gynaecologist.

© Dumas Ltd 2006

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