A chorionic villi sampling is usually done between 10 to 12 weeks of pregnancy and can be done either vaginally or through the abdomen. An ultrasound is used to locate the placenta, and either a small catheter or thin needle is used to remove a small sample of the outer layer of the placenta -- the chorionic villi. The small piece of the placenta is sent to a cytogenetics laboratory for chromosome analysis either by FISH analysis or by karyotyping. The placenta and the fetus have come from the same fertilized egg so the chromosomes in the placenta are the same as the chromosomes in the fetus. Chromosome analysis will reveal if the fetus has Down syndrome .
There is about a 1 in 400 chance that a CVS test can cause a miscarriage.