These tests are used to diagnose or rule out genetic abnormalities in the unborn child. Small samples of tissue are taken from the pregnancy and analysed in a laboratory. Both tests provide the same information. Which test is chosen depends on the stage of pregnancy. CVS is done between 10 and 14 weeks. Amniocentesis between 15 and 22 weeks.
The samples are taken using a fine needle passed through the skin guided by the ultrasound equipment. The test takes 30 and 60 seconds and is usually less uncomfortable than a blood test.
WHY HAVE A CVS OR AMNIOCENTESIS?
The commonest reason for having one of these tests is to diagnose or rule out Down Syndrome. This is almost always because of a risk of more than 1 in 300 on screening using ultrasound and blood testing at 11 to 14 weeks ( see 11 to 14 week scan site). CVS is usually the test performed.
Other reasons include abnormalities on ultrasound either at the 11 to 14 week ultrasound usually leading to a CVS or at the 20 week ultrasound usually leading to an amniocentesis. Sometimes these tests are chosen because of an increased risk of inherited disorders such as cystic fibrosis or thalassemia.
WHAT HAPPENS ON THE DAY OF THE TEST?
No special preparation is needed. An ultrasound is performed first. The sample is then taken. After a short time of resting the ultrasound is repeated to confirm all is well. We recommend some-one comes with you and drives you home as occasionally you may feel a little faint afterwards. You should avoid strenuous exercise and heavy lifting for 24 hours after the test.
WHAT COMPLICATIONS MAY OCCUR FOLLOWING THE TESTS?
There is a risk of miscarriage with amniocentesis and CVS of 1 in 200. This is in addition to a background risk of miscarriage which exists independent of the procedure of between 5 and 10%.
Approximately, 1 in 10 women experience some spotting within 2 -3 days of a CVS. Spotting or bleeding after amniocentesis is rare. If this occurs an ultrasound is usually performed to check all is well. Most pregnancies continue normally after an episode of bleeding.
This is a rare complication which only occurs after amniocentesis. It occurs in approximately in every 100 amniocenteses. An ultrasound is usually performed to check that there is a normal amount of fluid remaining. Almost, all pregnancies with this complication proceed normally.
Mild discomfort in the skin where the needle is inserted is common as is mild period cramping in the first 24 – 48 hours.
WHEN WILL THE RESULTS BE AVAILABLE
The standard analysis of the material takes 10 – 14 days. The cells obtained have to grow in the laboratory until there are enough to test. This is the most detailed and comprehensive analysis which is always done following an amniocentesis or CVS. A quicker result can be obtained with an optional test called FISH (Fluorescent Insitu Hybrisation ). This gives a quick result ( generally within 24 hours) for Trisomy 21 (Down Syndrome) and two other common abnormalities. It also determines the sex. This test is always done in conjunction with the full 10 – 14 day analysis. It provides no additional information but provides the important information more quickly.