Chorionic villus sampling (CVS) is a test that can detect Down syndrome and other chromosomal abnormalities, but it cannot detect neural tube defects. A thin tube is inserted through the vagina and cervix, or through the abdomen via a needle, to take a sample of the chorionic villi, wisps of tissue that attach the placenta to the wall of the uterus.

CVS is done at ten or eleven weeks of pregnancy (one month sooner than amniocentesis) and results are available within a week. CVS has a higher risk of miscarriage than amniocentesis (between .5 and 1 percent, according to the March of Dimes). It is considered to be more than 99 percent accurate. The test is recommended if a first-trimester screening indicates a risk of chromosomal problems, if a woman is at a higher risk for these kinds of abnormalities, or if waiting for an amniocentesis (which is done later in pregnancy) is not advised.

The advantage of this test is that it is more sensitive in detecting chromosome problems. It also offers information early in pregnancy, when termination is medically less difficult and the woman is likely to have more privacy in making the decision (since it is less obvious she is pregnant and a termination would not be as noticeable).

About 1 in 200 women have a miscarriage as a result of this test. In 1 to 2 percent of cases, a chromosome problem called mosaicism may be found, in which some cells have normal chromosomes and some don't. In this case, the chromosomal abnormality may be confined to the placenta and not be present in the baby. Generally an amniocentesis must be subsequently performed to confirm this.