After chorionic villus sampling (CVS) has been carried out, the sample of chorionic villi will be taken to a laboratory so that the cells can be examined under a microscope.

The number of chromosomes in the cells can be counted, and the structure of the chromosomes can be checked for any abnormalities. If the CVS is being carried out to test for a specific genetic disorder, the cells in the sample can also be tested for this.

CVS is estimated to be about 99% accurate. However, it cannot test for every birth defect, and it may not give conclusive results. In about 1% of cases, the results of CVS cannot establish with certainty that the chromosomes in the foetus are normal.

If this happens, it may be necessary to have further tests, such as examining the chromosomes in cells from the parents. It may also be necessary to have amniocentesis (an alternative test in which a sample of amniotic fluid is taken from the mother) to confirm a diagnosis.

Test results

The first result should be available within a few days, and this will tell you whether a major chromosome problem has been discovered.

The full results, including smaller, rarer conditions, can take two to three weeks to come back, although if the test is looking for a specific disorder the results may take up to a month.

For most women who have CVS, the results of the procedure will be "normal". This means the baby will not have any of the disorders that were tested for.

However, it is occasionally possible to have a normal result but a baby that is born either with the condition that was tested for or with another genetic condition. This is because a normal test result cannot exclude every possible genetic disorder.

If your test is "positive", your baby has the disorder that was being tested for. If you receive a positive test result, the implications will be fully discussed with you. There is no cure for most chromosomal conditions. Therefore, you need to consider your options carefully. These will include:

• continuing with your pregnancy, while gathering information about the condition so that you are prepared for caring for your baby
• having an abortion 

If you are considering an abortion, talk to your GP or midwife. They can give you important information and advice.

For example, your options for abortion will depend on how many weeks pregnant you are when you make the decision. If you decide to end your pregnancy, you may wish to talk to a counsellor afterwards. Again, your GP or midwife can help you arrange this.

The organisation Antenatal Results and Choices also helps parents with any issues around antenatal testing and its implications.