Amniocentesis / Chorionic Villus Sampling (CVS)

Invasive testing for chromosomal defects
The Oaks Clinic provides invasive testing, both amniocentesis and chorion villus sampling (CVS). These tests are undertaken by consultants who routinely perform them within the NHS. The risk of miscarriage of these tests is 1% in this unit. Results are available within 5 working days and common side effects include slight soreness over the site of the needle insertion, cramps and occasional dampness later on in the case of amniocentesis or bleeding in the case of chorion villous sampling. Very rarely infection can arise; this usually occurs 3 days after the test.

An amniocentesis test is performed when a screen positive result for Downs Syndrome is obtained from either the 11-13 week scan or the 16 week blood test for Downs Syndrome. Some women may need a test for reassurance, either because they have had an affected pregnancy or have a deep anxiety of having one. Another reason for having a test is where an abnormality, which may be associated with chromosomal abnormalities, is found at the 20 week scan.

An amniocentesis test involves the insertion of a fine needle, under local anaesthetic, into the sack of fluid surrounding the baby (amniotic fluid). This is an aseptic procedure and the needle is inserted under direct ultrasound guidance avoiding the baby, placenta and umbilical cord. About 10 - 20ml of amniotic fluid is obtained and sent to the laboratory for analysis.

Chorionic Villus Sampling (CVS)
This type of test can be performed after 11 weeks gestation. This is the preferred method of testing for early diagnosis of chromosomal and some genetic defects. Like the amniocentesis it involves the insertion of a needle under local anaesthetic into the placenta and taking a small biopsy of the placenta. This is performed aseptically under direct ultrasound guidance. Like amniocentesis there is a risk of about 1%; most of the risk is at the time of the test. There is always a small risk of confusing results where the placental sample does not reflect the chromosomes of the fetus. In this case the results may be delayed or an amniocentesis may need to be performed.