Chorionic villus sampling (CVS) is an antenatal test that is used to determine whether or not the foetus is carrying any chromosomal abnormalities, such as Down's syndrome.
It is a very accurate test and can determine with great precision whether or not an abnormality is present.
The test involves taking a sample of chorionic villi, which is tissue from the placenta. Because the placenta and the foetus are made from the same cells, the chorionic villi will contain accurate information about the baby's genetic make-up.
CVS cannot be performed until 11 weeks of pregnancy, as there is some risk to the development of the foetus's limbs. After this time, the risk involved is miscarriage, although this only affects two in every 100 women who undergo CVS.
There are two methods of obtaining CVS. Between 11 and 13 weeks of pregnancy, it is normally done transvaginally, which means a sample is taken from the placenta via the woman's vagina and cervix.
From week 13 onwards, samples are taken via a small needle that is inserted through the abdomen and into the uterus. Local anaesthetic is used in this procedure.
Both methods are done using ultrasound to ensure that the instruments are located in the correct position, and the procedure is described as uncomfortable but not especially painful. Initial results from the sample are available within two to three days, and the full result is usually available within two weeks.