Chorionic villus sampling (CVS): when it is offered, what happens and the risks

Chorionic villus sampling is a diagnostic test, which means it can give a definite answer about certain conditions, unlike a screening test, which only estimates a chance. A small piece of tissue is removed from the placenta, the organ linking the mother's blood supply with the unborn baby. The cells in the placenta normally have the same chromosomes as the baby, so analysing them can show whether the baby has a chromosomal difference.

Because it is an invasive procedure, it is not used as a first test. People are usually offered it after a screening result or ultrasound finding suggests a higher chance of a condition. The decision is personal. The NHS stresses that you do not have to have CVS if it is offered, and that a midwife or doctor will explain what the test involves and the possible benefits to help you decide.

The NHS lists the situations in which CVS may be offered. They are an antenatal screening test suggesting the baby may be born with a condition such as Down's, Edwards' or Patau's syndrome, a previous pregnancy affected by a genetic condition, and a family history of a genetic condition such as sickle cell disease, thalassaemia, cystic fibrosis or muscular dystrophy.

A hospital factsheet from University Hospital Southampton makes the same point: CVS is an optional test, offered if an earlier screening test has suggested a higher chance of chromosome differences. If you are offered it, you can ask which result prompted the offer, what conditions the test can and cannot detect, and how long it will take to receive results.

Before the procedure the Southampton factsheet says you must have had a blood group test, so that anti-D can be offered if you are rhesus D negative, and screening for HIV and hepatitis B, so that precautions can be taken if either infection is known. You can eat and drink as normal. You are asked to come with a comfortably full bladder, which lifts the womb out of the pelvis and makes it easier to see the baby and placenta. An ultrasound scan is done first to measure the baby and check where the placenta is.

The NHS says a small sample is removed from the placenta using one of two methods. In transabdominal CVS, which is the most common, a needle is inserted through the tummy. In transcervical CVS, a tube or small forceps is inserted through the cervix. In the transabdominal method described by the Southampton factsheet, the abdomen is cleaned, local anaesthetic is injected into the skin, and a fine needle is passed into the placenta while ultrasound is used continuously to monitor the position of the baby and placenta.

The NHS says the test itself takes about 10 minutes, though the whole consultation may take about 30 minutes. You may have cramps similar to period pains for a few hours afterwards.

The main risk is miscarriage, which the NHS defines here as loss of the pregnancy in the first 23 weeks. It says the chance is thought to be less than 1 in 200 for most pregnancies and around 1 in 100 for multiple pregnancies. The Southampton factsheet gives approximately one in every 200 women who have CVS. You may also get an infection, or need to have the test repeated because it was not successful the first time.

After the test, the Southampton factsheet recommends that someone drives you home. You can return to a normal level of daily activity as soon as you feel able, as there is no evidence to support bed rest or avoiding strenuous activity. Mild discomfort can be treated with paracetamol. It says to contact the fetal medicine team or your local maternity hospital immediately if you have severe pain, bleeding, loss of vaginal fluid or flu-like symptoms.

The NHS says the first results, known as the rapid CVS result, should be available in about 3 days, and a more detailed set after 2 weeks. The Southampton factsheet, which describes a QF-PCR test, says the result is usually available two to three working days after the laboratory receives the sample.

A QF-PCR test looks for whole extra chromosomes. The factsheet lists its limits: it may not pick up very subtle chromosome imbalances, does not provide information about tiny changes in individual genes so may not pick up all genetic conditions, does not test for spina bifida or other physical differences, and cannot detect conditions such as autism or cerebral palsy. It may also find a chromosome change that is hard to interpret.

Occasionally the result is unclear. The factsheet explains that this can be because of a rare condition called confined placental mosaicism, in which the DNA in the placenta is not the same as the baby's. An amniocentesis may then be offered. If a condition is found, the NHS says a specialist doctor or midwife will explain the results and discuss your options, which may include continuing the pregnancy while gathering information or considering ending it. Support organisations such as Antenatal Results and Choices and the Down's Syndrome Association are listed in the factsheet.

The NHS describes amniocentesis as the alternative to CVS. A small sample of the fluid surrounding the baby is removed, usually between the 15th and 18th weeks of pregnancy, although it can be later if necessary. It can also cause a miscarriage, and because it is done later in pregnancy, there is less time to consider options once the result comes back. The specialist carrying out the test can discuss the different options and help you decide.