Chorionic villus sampling (CVS) is a highly accurate prenatal test
that detects chromosomal abnormalities such as Down syndrome. Its
central advantage over amniocentesis is that you can take it earlier
between the tenth and 12th weeks of pregnancy.
You can't have an
amnio until your second trimester. Many of the women who opt for the
test are over age 35 (which means they have one chance in 134 of
carrying a child with a chromosomal problem), or have a child with a
birth defect, or a family history of a genetic disease such as Tay
Sachs. Until recently, only a few medical centers offered the CVS
test.
But enough technicians are now familiar with the procedure that
it has become a popular alternative to amniocentesis.
National Geographic - In the Womb
Paralumun New Age Village