WebMD explains chorionic villus sampling (CVS), a prenatal test that helps detect birth defects, genetic diseases, and other problems early in a pregnancy.

Each villus (individual villi) also has tiny microvilli, which are even smaller projections that also aid digestion. Tiny depressions called "crypts" separate the villi.

Chorionic villus sampling is a prenatal diagnostic test performed for prenatal genetic testing involving a biopsy of placental tissue between 10 to 13 weeks gestation.

Villi can absorb nutrients through their tops, sides, and bases, dramatically increasing the area dedicated to nutrient absorption. In a human, each villus is about 1 millimeter long, or roughly 13 ...

The risks associated with chorionic villus sampling pose a dilemma for doctors who are learning procedure, writes Adam Wolfberg, M.D.