A diagnostic procedure for pregnant women to determine whether or not their fetuses have gross chromosomal anomalies. It involves collecting a small sample of chorion cells for karyotyping. The biopsy usually is done by inserting a small flexible plastic tube through the vagina and the cervix of the uterus to draw out a sample of chorion tissue. Alternately, the cells may be extracted with a hypodermic needle through the uterine wall, as in the case of amniocentesis. With CVS, there is approximately 98% accuracy in diagnosing Down syndrome and many other conditions due to gross chromosomal abnormalities. However, the accuracy in predicting neural tube defects is lower than with amniocentesis. Some sources refer to CVS as "chorionic villus sampling". "Villus" is the singular of "villi".