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

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, or for women in whom the fetal paternity is in doubt.

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 11 and 14 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
  • 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. 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

The test takes about 15 minutes and the results should be available within 10-14 days for a paternity test or within 48 hours for a genetic test.

CVS is not entirely without risk. In around two per cent of cases (1 in 50), the procedure can result in a miscarriage (the loss of the pregnancy).

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