Procedure: Chronic Villi Sampling

Description and Purpose

Chronic or chorionic villus sampling , is where, in order to obtain cells from the placenta surface, a thin catheter is inserted into the cervix and uterus and a sampling of cells is suctioned into the tube. Ultrasound is used to guide placement of the catheter. The purpose of this procedure includes:

• To detect suspected genetic abnormalities in the foetus.
• To identify chromosome disorders, most commonly Down syndrome in women over 35.

Preparation and Procedure

If the test is undertaken in the first trimester, a full bladder is required, as it helps push the uterus out of the pelvis and thus gives a better view.
An ultrasound is used to observe the position of the uterus and placenta, both of which help determine whether a catheter will be inserted through the vagina and cervix or a needle inserted through the abdomen and uterine wall. If the sample is taken through the uterine wall, a local anaesthetic may be applied first.
For the needle method, an ultrasound transducer is placed on the abdomen as in amniocentesis .
For the catheter method, a thin metal rod is inserted into the cervix to help identify it on the ultrasound scan and show how best to insert the catheter into the uterus.

	Advantages Disadvantages Can be performed earlier in pregnancy than amniocentesis (at around ten weeks). It is almost 100% reliable in detecting chromosomal and genetic defects. It does not measure alpha-fetoprotein (AFP), so a supplemental blood test must be used. It entails a slightly higher risk of miscarriage than does amniocentesis (1%-2% greater). It's less commonly available than amniocentesis, and fewer doctors are experienced in the procedure. It entails a greater risk of distorted results than does amniocentesis due to presence of mother's cells in the sample and discrepancies between chorionic villi and foetal genes. Metabolic disorders are difficult to diagnose and must be confirmed with amniocentesis. Because of the early gestational age at which the test is performed, foetal anatomy cannot be seen as well as it can at the time amniocentesis is performed.

The information in this page is presented in summarised form and has been taken from the following source(s):
1. The Yale University School of Medicine Patient's Guide to Medical Tests, Barry L. Zaret M.D., Senior Editor, published by Houghton Mifflin. Online: http://www.collectivemed.com/jump/mtest.shtml