chorionic villus sampling (CVS) or chorionic villus biopsy (CVB) [key], diagnostic procedure in which a sample of chorionic villi from the developing placenta is removed from the uterus of a pregnant woman (see pregnancy) using a fine needle inserted through the abdomen or a thin plastic catheter inserted into the vagina and through the cervix. Chorionic villi are fingerlike projections of a membrane (the chorion) that surrounds the fetus. The villi develop from the fertilized ovum, or egg, and have a genetic composition similar to that of the fetus. Cells in the sample are grown in the laboratory and studied to detect the presence in the fetus of such genetic birth defects as Tay-Sachs disease and Down syndrome. The sex of the child can also be ascertained. Although CVS tests for the same range of abnormalities as amniocentesis, it is usually performed some weeks earlier (between the 8th and 12th weeks of pregnancy), and the results are available in a few days. It is recommended if the parents are carriers of certain genetic diseases, if there is a family history of genetic disorders, or if the woman is over age 35 (later pregnancies carrying with them a higher risk of chromosomal abnormality).
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