Chorionic Villus sampling

Chorionic villus sampling (CVS) is a test that is carried out during pregnancy to detect serious problems with the foetus. 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 (inherited) defects.

CVS is available to women who are at greater risk of having a baby with an inherited disorder, such as Down’s syndrome or muscular dystrophy. This could be because:

• there is a family history of the condition, or
• the pregnant woman is over 35 years of age.

CVS is an alternative to amniocentesis (where a sample of the mother's amniotic fluid is taken for testing) and can be carried out earlier in the pregnancy. CVS is usually carried out between weeks 10 and 13 of the pregnancy, while amniocentesis cannot be carried out until week 15. 

What are chorionic villi? 

At an early stage of pregnancy, the embryo divides into two parts:

• one part develops into the baby, and
• the other part develops into the placenta.

The part of the embryo that forms the placenta starts out as finger-like sections that are called chorionic villi. These burrow into the wall of the womb (uterus) to get close to the mother's blood vessels.

The chorionic villi are formed by the division of the fertilised egg, which means they have exactly the same DNA (genetic code) as the embryo, including any possible genetic abnormality. Any defect in the chorionic villi will also be present in the foetus.

The procedure

During CVS, a sample of chorionic villi cells will be taken from the pregnant woman’s placenta using either:

• transabdominal CVS - where a needle is inserted through the abdomen, or
• transcervical CVS - a tube inserted through the cervix (the neck of the womb). 

The test takes about 20 minutes and the results should be available within10-14 days.

CVS is not entirely without risk. In around two per cent of cases, the procedure can result in a miscarriage (the loss of the pregnancy).