Chorionic villus sampling (CVS) is a test done during early pregnancy that can find certain problems with an unborn baby (fetus). It may be done when either parent has a family history of an inherited genetic disorder or when the mother wants to find out her chances of having a baby with a chromosome problem.

Chorionic villi are tiny fingerlike projections found in the placenta. The genetic material in chorionic villus cells is the same as in fetal cells. During CVS, a sample of chorionic villus cells is taken and examined for chromosome problems, such as Down syndrome. The sample is usually collected by putting a thin flexible tube (catheter) through the vagina and cervix into the placenta (transcervical CVS). The sample can also be collected by putting a long, thin needle through the belly into the placenta (transabdominal CVS). Ultrasound is used to guide the catheter or needle into the correct position for collecting the sample.

CVS is usually done late in the first trimester, between the 10th and 12th weeks of pregnancy.