Chorionic Villos Sampling, known as CVS, like amniocentesis, is used to detect fetal defects. Unlike amniocentesis, CVS is done around 8 - 9 weeks of pregnancy. The benefit is that if termination of the pregnancy is required, it can be done at a much earlier and safer a time. However, at present, CVS has about a 2-3 times greater risk of miscarriage than amniocentesis. The risk may well decrease as the procedure is done more.
The procedure takes technical skill. A long, thin tube is placed into the uterus through the vagina. This is done under direct visualization with the ultrasound. A sampling of tissue known as chorionic villi, is removed. The chorionic villi are finger-like projections from fetal membranes which will form part of the placenta. This tissue normally disappears as the fetus grows.
It is believed that there is no danger in taking samples of this tissue, but at present, CVS should be done only when absolutely indicated.