CVS is normally performed between 11 - 13 weeks of pregnancy. A needle is inserted through the mother's abdomen or cervix and is used to remove a small piece of the placenta (afterbirth). Ultrasound scanning is used to locate the baby and the placenta and to guide the needle.
The placenta has the same chromosomes and therefore genetic makeup as the baby. The cells are analysed in a laboratory and the baby's chromosomes are counted.
This procedure carries a risk of miscarriage of between 1 - 2%.
